Properly fix or delete OwnCloudExportTest

[djbrown]

The ownCloud demo server was shut down.

OwnCloudExportTest has only one test case. It references the former demo server:

gnucash-android/app/src/androidTest/java/org/gnucash/android/test/ui/OwnCloudExportTest.java

Line 83 in 39ebc89

private String OC_SERVER = "https://demo.owncloud.org";

In #155 the test was modified to abort before connecting to the demo server.

The goal of this issue is to handle the test properly.

Possible solutions:

1. delete the test (file)
2. refactor for testability (dependency inversion) and pass implementation with static test behavior
3. mock (override) existing production methods (using e.g. mockito)
4. provide a mock server with static test behavior (using e.g. wiremock)
5. setup a real ephemeral ownCloud instance within GitHub workflow and locally (using e.g. docker)
6. setup a real persistant ownCloud instance with separate infrastructure (using e.g. google cloud platform)