CI: Split macOS Linux boot tests into parallel jobs

[ian0318git]

This PR splits the macOS guest Linux boot tests into dedicated parallel matrix jobs, mirroring the existing setup for Linux hosts (boot-tests-x64).

Fixes #792

Changes:

• Extracted sequential boot Linux kernel test steps from macOS-arm64 job.
• Added new boot-tests-macos job with matrix covering both gcc-15 and clang across interpreter and jit configurations.
• Adjusted job timeout and boot timeouts accordingly.

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Summary by cubic

Splits the macOS guest Linux boot tests into dedicated parallel matrix jobs so they no longer run sequentially inside macOS-arm64, matching the existing boot-tests-x64 setup for Linux hosts. Fixes #792.

• Adds a new boot-tests-macos job covering gcc-15 and clang across interpreter and jit builds.
• Removes the sequential boot test steps from the macOS-arm64 job.
• Sets per-job and boot timeouts, with a higher boot timeout for JIT to avoid flakes under macOS load.

^{Written for commit 8b81d41. Summary will update on new commits.}

[jserv]

Avoid mentioning (#792) in the subject.
Read https://cbea.ms/git-commit/ carefully and enforce the rules.

[ian0318git]

Closing in favor of parallelized solution in #797.