test: Add test codebase for shell completions

[shannmu]

What does this PR try to resolve?

Tracking issue https://github.com/rust-lang/cargo/issues/14520

• Refactor candidates generators to make them testable
• Add a test codebase and template project used for building test cases
• Add test code for get_registry_candidates, get_example_candidates, get_test_candidates, get_bench_candidates, get_bin_candidates

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[rustbot]

:umbrella: The latest upstream changes (possibly 307cbfda3119f06600e43cd38283f4a746fe1f8b) made this pull request unmergeable. Please resolve the merge conflicts.

[weihanglo]

@rustbot author

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