CI: switch to GitHub Actions - step 2: test stage

This commit:
* Adds a GH Actions workflow for the CI check which was previously run on Travis in the `test` stage.
* Removes the, now redundant, `.travis.yml` configuration.
* Updates the `.gitattributes` file.

Notes:
1. For the time being, I've commented out the job against PHP 8.1/nightly as - no matter what I've tried - I can't even get the tests running at the moment, so they would only ever fail.
    Once PHP 8.1 starts to release alpha/beta/RC builds, further testing should be done to attempt to get this working.
    Notes/current findings:
    - With PHPUnit 7.x on PHP 8.1, using a configuration file will block the tests from running.
    - In some cases (PHPCS itself, PHPCSExtra), I managed to get the tests running by passing all relevant information via the command-line and actively ignoring the configuration file using `--no-configuration`. Unfortunately, that does not seem to work in this case. Further investigation is needed.
    - Based on the (running) tests for PHPCSExtra on PHP 8.1, an incompatibility with PHP 8.1 in PHPCS itself has already been discovered.
        A fix for this has been pulled, but not yet merged: squizlabs/php_codesniffer 3250

.gitattributes

@@ -8,7 +8,6 @@
 /.gitattributes export-ignore
 /.gitignore export-ignore
 /.phpcs.xml.dist export-ignore
-/.travis.yml export-ignore
 /phpunit.xml.dist export-ignore
 /.github export-ignore
 /bin export-ignore

.github/workflows/test.yml

@@ -0,0 +1,101 @@
+name: Test
+
+on:
+  # Run on all pushes and on all pull requests.
+  # Prevent the "push" build from running when there are only irrelevant changes.
+  push:
+    paths-ignore:
+      - '**.md'
+  pull_request:
+  # Allow manually triggering the workflow.
+  workflow_dispatch:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+
+    strategy:
+      # Keys:
+      # - php: The PHP versions to test against.
+      # - phpcs_version: The PHPCS versions to test against.
+      #   IMPORTANT: test runs shouldn't fail because of PHPCS being incompatible with a PHP version.
+      #   - PHPCS will run without errors on PHP 5.4 - 7.4 on any supported version.
+      #   - PHP 8.0 needs PHPCS 3.5.7+ to run without errors.
+      # - The `wpcs_version` key is added to allow additional test builds when multiple WPCS versions
+      #   would be supported. As, at this time, only the latest stable release of WPCS is supported,
+      #   no additional versions are included in the array.
+      # - experimental: Whether the build is "allowed to fail".
+      matrix:
+        php: ['5.4', '5.5', '5.6', '7.0', '7.1', '7.2', '7.3', '7.4']
+        phpcs_version: ['3.5.5', 'dev-master']
+        wpcs_version: ['2.3.*']
+        experimental: [false]
+
+        include:
+          # Complete the matrix by adding PHP 8.0, but only test against compatible PHPCS versions.
+          - php: '8.0'
+            phpcs_version: 'dev-master'
+            wpcs_version: '2.3.*'
+            experimental: false
+          - php: '8.0'
+            # PHPCS 3.5.7 is the lowest version of PHPCS which supports PHP 8.0.
+            phpcs_version: '3.5.7'
+            wpcs_version: '2.3.*'
+            experimental: false
+
+          # Experimental builds. These are allowed to fail.
+          #- php: '8.1'
+          #  phpcs_version: 'dev-master'
+          #  wpcs_version: '2.3.*'
+          #  experimental: true
+
+    name: "Test: PHP ${{ matrix.php }} - PHPCS ${{ matrix.phpcs_version }} - WPCS ${{ matrix.wpcs_version }}"
+
+    continue-on-error: ${{ matrix.experimental }}
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      # On stable PHPCS versions, allow for PHP deprecation notices.
+      # Unit tests don't need to fail on those for stable releases where those issues won't get fixed anymore.
+      - name: Setup ini config
+        id: set_ini
+        run: |
+          if [[ "${{ matrix.phpcs_version }}" != "dev-master" ]]; then
+            echo '::set-output name=PHP_INI::error_reporting=E_ALL & ~E_DEPRECATED'
+          else
+            echo '::set-output name=PHP_INI::error_reporting=E_ALL'
+          fi
+
+      - name: Install PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          ini-values: ${{ steps.set_ini.outputs.PHP_INI }}
+          coverage: none
+
+      - name: 'Composer: set PHPCS and WPCS versions for tests'
+        run: |
+          composer require --no-update --no-scripts squizlabs/php_codesniffer:"${{ matrix.phpcs_version }}"
+          composer require --no-update --no-scripts wp-coding-standards/wpcs:"${{ matrix.wpcs_version }}"
+
+      # Install dependencies and handle caching in one go.
+      # @link https://github.com/marketplace/actions/install-composer-dependencies
+      - name: Install Composer dependencies - normal
+        if: ${{ startsWith( matrix.php, '8' ) == false }}
+        uses: "ramsey/composer-install@v1"
+
+      # PHPUnit 7.x does not allow for installation on PHP 8, so ignore platform
+      # requirements to get PHPUnit 7.x to install on nightly.
+      - name: Install Composer dependencies - with ignore platform
+        if: ${{ startsWith( matrix.php, '8' ) }}
+        uses: "ramsey/composer-install@v1"
+        with:
+          composer-options: --ignore-platform-reqs
+
+      - name: Run the unit tests
+        run: ./bin/unit-tests
+
+      - name: Run the ruleset tests
+        run: ./bin/ruleset-tests