Add exclude-test filter flag

[TUPM96]

Summary

• Add a --exclude-test detector flag that appends a default test/mock path filter.
• Allow combining it with existing --filter-paths, while rejecting --include-paths because include filtering takes the opposite path.
• Cover the helper and default pattern with unit tests.

Fixes #691

Testing

• uv run pytest tests/unit/utils/test_command_line.py
• uv run ruff check slither/main.py slither/utils/command_line.py tests/unit/utils/test_command_line.py
• codespell slither/main.py slither/utils/command_line.py tests/unit/utils/test_command_line.py
• git diff --check

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