test: expand gtest coverage for common/core utilities
Add unit tests for:
- CommandLineParser, Debug handler, Decibel, Digit, JobTime, FFmpegUtils
- Core StringUtils, Timecode, TimeRange, Bezier, Color
- Extend Frame tests to cover pixel access, interlace, conversion
Coverage improvements (app + ext/core):
- commandlineparser.cpp: 0% -> 93%
- debug.cpp: 0% -> 85%
- decibel.h: 25% -> 88%
- digit.h: 0% -> 100%
- ffmpegutils.cpp: 29% -> 90%
- frame.cpp: 53% -> 97%
- bezier.cpp: 16% -> 98%
- color.cpp: 13% -> 71%
- Overall line coverage: 17% -> 19%
Mike-Solar
c57608b8f8
