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
• 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".

[pont-us]

Simple and effective. Approved with a minor language nitpick.

[codecov]

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 ⚠️

Additional details and impacted files

@@            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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[TonioF]

I see this rather as a workaround than a proper fix. In line 59, we instantiate an AnyThreadEventLoopPolicy which has been deprecated (see https://www.tornadoweb.org/en/stable/asyncio.html#tornado.platform.asyncio.AnyThreadEventLoopPolicy). I assume we'd need to remove this and that we still will encounter issues in the future. I approve this PR as it solves our momentary problems, but I suggest not to close the issue.