Use google test for test cases instead of QtTest.
Rewrote first test suite with google test.
Rewrite second test suite with Google test.
Rewrote third test suite with google test.
Rewrote last test suite with Google test.
Merge pull request #58 from DO-CV/maint-painting-window-test

[MAINT] Rewrite unit tests for `PaintingWindow` with Google Test.
Remove unneeded include.
Merge pull request #59 from DO-CV/maint-painting-window-test-bis

[MAINT] Remove unneeded include file in unit test.
Merge remote-tracking branch 'origin' into maint-add-unit-test-for-opengl-window