gh-131178: Add tests for quopri command-line interface

[Ar-maan05]

Adds a new CommandLineTest class in Lib/test/test_quopri.py covering behavior that was previously untested:

• encode/decode with file-argument input
• -t (quote tabs)
• explicit - for stdin
• multiple input files
• -t and -d are mutually exclusive (exit code 2)
• unknown short option (exit code 2, usage on stderr)
• nonexistent file (exit code 1, diagnostic on stderr)

The pre-existing test_scriptencode / test_scriptdecode in QuopriTestCase are left in place.

• Issue: Add tests for command-line interface #131178

[python-cla-bot]

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[Ar-maan05]

Friendly ping! Let me know if any changes are needed.

[github-actions]

This PR is stale because it has been open for 90 days with no activity.