Create test vector code coverage reports

[jzern]

https://llvm.org/docs/CommandGuide/llvm-cov.html

Knowing what areas of the decoder are covered by the test vectors will help identify what areas are missing and ensure the current files are testing the areas they were intended to. For that, an overall report, using all the test vectors, and per-file reports might be helpful to move the work in libiamf forward.