* Migrate from CppUnit to gtest
- include GoogleTest to cmake files and remove CppUnit
- include GoogleTest in azure-pipelines and remove CppUnit
- move tests from test/ to test/unit_tests/ and adapt to GoogleTest
- add two testcases from XojPreviewExtractorTest that were previously
skipped and fix them
- pin GoogleTest version to git tag "release-1.11.0`
- bump required CMake version to 3.14 to allow for `FetchContent_MakeAvailable`
- set `test-units` target to `EXCLUDE_FROM_ALL`
- fix coverage not properly working because of missing link options
- Improve `rpath` configuration in `test/CMakeLists.txt` to no longer require changing the `rpath` after build
* Add extensive documentation under `test/README.md`
* Incorporate several Review Comments