Reduce platform-gated test specs

[MikeMcQuaid]

• replace :needs_linux and :needs_macos where Homebrew::SimulateSystem already drives the behaviour under test
• exercise launchctl and systemctl discovery through temporary PATH entries so service specs run on both platforms
• add reset_launchctl! so the tests reset cached state without reaching into instance variables

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OpenAI Codex with manual review and tweaks.

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[Copilot]

Pull request overview

This PR reduces platform-gated specs by using Homebrew::SimulateSystem and temporary PATH shims to make the relevant tests exercise discovery logic on both macOS and Linux, while adding a small reset hook to avoid poking cached state via instance variables.

Changes:

• Replace :needs_linux/:needs_macos in several specs with Homebrew::SimulateSystem.with(...).
• Update services-related specs to discover launchctl/systemctl via temporary executables in PATH, enabling cross-platform execution.
• Add Homebrew::Services::System.reset_launchctl! to reset cached launchctl discovery between tests.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
Library/Homebrew/test/software_spec_spec.rb	Removes platform gating by simulating OS versions and updating expectations around uses_from_macos.
Library/Homebrew/test/services/system_spec.rb	Makes launchctl/systemctl discovery tests portable via temporary PATH executables and cache resets.
Library/Homebrew/test/services/system/systemctl_spec.rb	Reworks .executable spec to use a temporary systemctl on PATH instead of platform gating.
Library/Homebrew/test/services/cli_spec.rb	Exercises systemd_load/launchctl_load by running temporary shim executables and asserting captured arguments.
Library/Homebrew/test/service_spec.rb	Replaces platform-gated service command selection specs with Homebrew::SimulateSystem contexts.
Library/Homebrew/services/system.rb	Adds reset_launchctl! to reset cached launchctl path.

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