Add 2FA test and 2FA support for integration tests

[FranciscoPombal]

• Credentials for non 2FA/MFA-enabled accounts as well as 2FA/MFA-enabled accounts are supplied from the environment.

• For tests that can use both types of credentials, credentials for non 2FA/MFA-enabled accounts are preferred, if available.

• Tests that require one or the other type of credentials/accounts are skipped if the required type of credentials is not available.

• A new test-only dependency is introduced for generating 2FA/MFA codes from their secrets: github.com/pquerna/[email protected].

• golang.org/x/crypto has been bumped to the latest version.

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This is not ready because I cannot get the tests to pass properly.
With a 2FA/MFA-enabled account, I can get all tests to pass except one if I:

• Run them one at a time.
• Wait a bit between running each test (a few tens of seconds).
• Clear the login sessions created by the tests in the MEGA web app's "Session history" interface between each test.

When these workarounds are not all performed, the tests will fail with the following error: The upload target URL you are trying to access has expired. Please request a fresh one.
Additionally, TestEventNotify is not passing no matter what, with the same error.

I have not tested with a non 2FA/MFA-enabled account.

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Currently my knowledge of MEGA's API and SDK internals is limited, so any help fixing these issues is appreciated.
I would expect we need to at least clean up sessions between tests (perform a logout).

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Test setup to replicate the above results (for a 2FA/MFA-enabled account:

1. Export these variables to the environment:

   [email protected]
   MEGA_PASSWD_MFA=foobar123
   MEGA_SECRET_MFA=SECRETTOKEN2FAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA====
   

2. Run tests.

   Note that you may need to increase go test's -timeout if the MEGA account user for testing has many files, because for such accounts it may take a long time to login.
   This also explains the decision to pass the 2FA/MFA secret and calculate the codes just-in-time for each test instead of passing in a code for all tests.
   With a large account, some tests can take more than 30s to run, causing the remaining tests to be skipped due to any single code becoming invalid in the meantime.

[ncw]

I would expect we need to at least clean up sessions between tests (perform a logout).

We really should be doing this anyway as the sessions just pile up. Rclone now has a Shutdown method which could be used for this.

Currently my knowledge of MEGA's API and SDK internals is limited, so any help fixing these issues is appreciated.
I would expect we need to at least clean up sessions between tests (perform a logout).

My knowledge is limited too! I took on a maintainer role for this repo so I could keep it working with rclone, but I don't have time to dig into the SDK to find out how stuff works. I think you know more about it than me now!

These tests look safe to merge as they won't run unless we have the correct env var.