test/bundle/formula_installer_spec: avoid slow brew link

[cho-m]

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We probably should look into improving performance here as brew bundle can now directly interact with Homebrew::Cmd::Link. So it should no longer need to create another brew instance, which requires subshell and then another Ruby adding a noticeable overhead.

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However, in the mean time, we can avoid running this code for testcases that aren't verifying link behavior providing a significant speed up on one of slowest test files:

Before:

Top 3 slowest examples (15.17 seconds, 26.6% of total time):
  Homebrew::Bundle::FormulaInstaller when the formula is installed when the postinstall option is provided when formula has changed reports a failure
    5.13 seconds ./test/bundle/formula_installer_spec.rb:246
  Homebrew::Bundle::FormulaInstaller when the formula is installed with an always restart_service option with a skipped installation restart service
    5.02 seconds ./test/bundle/formula_installer_spec.rb:93
  Homebrew::Bundle::FormulaInstaller when the formula is installed when the postinstall option is provided when formula has changed runs the postinstall command
    5.02 seconds ./test/bundle/formula_installer_spec.rb:240

Took 58 seconds

After:

Top 3 slowest examples (1.3 seconds, 8.2% of total time):
  Homebrew::Bundle::FormulaInstaller.outdated_formulae calls Homebrew
    0.49262 seconds ./test/bundle/formula_installer_spec.rb:317
  Homebrew::Bundle::FormulaInstaller#restart_service_needed? is false by default
    0.46559 seconds ./test/bundle/formula_installer_spec.rb:559
  Homebrew::Bundle::FormulaInstaller#start_service_needed? when a service is already started is false with {restart_service: :always}
    0.34286 seconds ./test/bundle/formula_installer_spec.rb:502

Took 16 seconds

[Copilot]

Pull request overview

This PR improves test performance by avoiding slow brew link subprocess calls in tests that don't verify linking behavior. The changes mock Homebrew::Bundle.brew("link", ...) in three test contexts that focus on service management and postinstall behavior, reducing the test file execution time from 58 seconds to 16 seconds.

• Adds mocking to avoid subprocess overhead in non-link-related tests
• Achieves ~72% reduction in test execution time (58s → 16s)
• Follows established testing patterns by using allow stubs for non-verified behavior

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[MikeMcQuaid]

Great work!