Add help screen to slang-test (#6611)

This commit adds a help screen to slang-test to improve usability;
and update README.md with clearer instructions.

The help screen is displayed when:
- User explicitly requests help with -h or --help flags
- An unknown option is provided
- A required argument for an option is missing

The help screen provides comprehensive documentation of all available options,
organized into sections:
- Basic options (e.g. bindir, test-dir)
- Output modes (e.g. appveyor, travis, teamcity)
- Test prefix usage explanation

Fixes: #6560

Co-authored-by: Ellie Hermaszewska <ellieh@nvidia.com>

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