Test coverage improvements

[swissspidy]

Description

This PR makes some of the test code more DRY by adding a new TestCase class. In the future this class can be extended to add helper factories to create projects or create some entities by default.

Also improves test coverage for some parts of the plugin.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code has proper inline documentation.

[codecov-io]

Codecov Report

Merging #118 into master will increase coverage by 1.25%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #118      +/-   ##
============================================
+ Coverage     76.41%   77.66%   +1.25%     
+ Complexity      341      328      -13     
============================================
  Files            23       23              
  Lines           814      788      -26     
============================================
- Hits            622      612      -10     
+ Misses          192      176      -16

Impacted Files	Coverage Δ	Complexity Δ
inc/WebhookHandler/GitLab.php	100% <ø> (+3.03%)	10 <0> (-1)	⬇️
inc/Repository/Bitbucket.php	96.77% <100%> (-0.85%)	19 <0> (-5)
inc/Repository/GitHub.php	100% <100%> (ø)	5 <0> (-5)	⬇️
inc/Repository/Base.php	87.5% <100%> (+16.53%)	25 <5> (+3)	⬆️
inc/Project.php	100% <100%> (+3.44%)	44 <1> (ø)	⬇️
inc/Plugin.php	29.6% <100%> (+4.8%)	39 <0> (ø)	⬇️
inc/Repository/GitLab.php	100% <100%> (+4%)	6 <0> (-5)	⬇️
inc/WebhookHandler/Bitbucket.php	97.29% <0%> (+5.4%)	12% <0%> (ø)	⬇️
... and 1 more

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