Improve CI testing

[romangg]

• Run tests also on release build.
• Enable Unity Build for test targets. This reduces the compile time by around 40%.
• Run Clang-Tidy with current config.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 93.50766% with 72 lines in your changes are missing coverage. Please review.

Project coverage is 50.80%. Comparing base (5f25519) to head (e7c856e).

Files	Patch %	Lines
tests/integration/lockscreen.cpp	0.00%	21 Missing :warning:
tests/integration/opengl_shadow.cpp	0.00%	15 Missing :warning:
tests/integration/effects/minimize_animation.cpp	0.00%	8 Missing :warning:
tests/integration/effects/slidingpopups.cpp	0.00%	7 Missing :warning:
...ntegration/effects/window_open_close_animation.cpp	0.00%	6 Missing :warning:
tests/integration/no_crash_no_border.cpp	0.00%	6 Missing :warning:
tests/integration/buffer_size_change.cpp	0.00%	2 Missing :warning:
tests/integration/decoration_input.cpp	66.66%	0 Missing and 2 partials :warning:
...tegration/effects/subspace_switching_animation.cpp	0.00%	2 Missing :warning:
...s/integration/no_crash_reinitialize_compositor.cpp	0.00%	2 Missing :warning:
... and 1 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #20 +/- ## ========================================== + Coverage 50.02% 50.80% +0.78% ========================================== Files 938 938 Lines 87251 87395 +144 Branches 45466 45307 -159 ========================================== + Hits 43645 44402 +757 + Misses 38496 38087 -409 + Partials 5110 4906 -204 ```

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