CI: Add macos-15 runner to test matrix

[PatTheMav]

Description

Adds macos-15 to the architecture matrix of the tests workflow.

Motivation and Context

Ensures that precompiled packages exist for macos-15 runners used by projects that also use this Homebrew tap for access to pinned clang-format versions.

How Has This Been Tested?

Needs successful CI run for tests.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.