fixing GitHub CI #899

[thomasstorm]

This PR fixes #899. The reason for the test failures in GitHub has been a race condition: the server starts in the background, and on another thread, the tests use it. The fix ensured that the server has started before the setUp() method terminates.

The checklist items below do not apply mostly, as there are no new features to be documented.

Checklist:

• [ ] Add unit tests and/or doctests in docstrings
• [ ] Add docstrings and API docs for any new/modified user-facing classes and functions
• [ ] New/modified features documented in docs/source/*
• [ ] Changes documented in CHANGES.md
• [x] GitHub CI passes
• [ ] AppVeyor CI passes
• [ ] Test coverage remains or increases (target 100%)

[pont-us]

I edited the title, since I assume "#886" was a typo for "#899".

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (3bec012) 93.43% compared to head (7102fbb) 93.43%.

Files	Patch %	Lines
xcube/server/testing.py	75.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #918 +/- ## ========================================== - Coverage 93.43% 93.43% -0.01% ========================================== Files 453 453 Lines 35510 35518 +8 ========================================== + Hits 33180 33186 +6 - Misses 2330 2332 +2 ```

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