test/formula_auditor_spec: add Linux-only GCC dep tests

[cho-m]

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Tests for

• formula_auditor: ignore implicit dep for Linux gcc audit #21765

[Copilot]

Pull request overview

Adds RSpec coverage for Homebrew::FormulaAuditor#audit_gcc_dependency, validating the Linux-only GCC dependency audit behavior introduced/adjusted in #21765.

Changes:

• Add specs ensuring the audit is skipped when not running on Linux.
• Add specs that flag Linux-only depends_on "gcc" in homebrew/core on Linux.
• Add specs covering allowlisted exceptions, implicit GCC deps, non-core taps, and non-OS-specific GCC deps.

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[MikeMcQuaid]

Makes sense!