Test coverage

[weichch]

We should add more tests and make sure at least 90% (currently 83% in main package, 66%-78% in assert packages) overall test coverage.

• Add more test cases
• Make sure overall test coverage is at least 90%
• Exclude external code (such as Google's diff_match_patch)
• Fix test coverage for .NET Standard 2.0 (currently only .NET Framework 4.6.1 and .NET Standard 2.1 are covered)
• Calculate test coverage in Github Action if possible
• Export test results when needed