Tests: Update attr_inlinable_available.swift to require macOS and remove the OS versions for other platforms from availability attributes.

Upcoming changes to the diagnostics make it difficult to verify correct behavior when only matching a fraction of a diagnostic, so the test needs to run on a single platform in order to have stable diagnostics to match. The `availability_refinement_contexts_target_min_inlining.swift` test case runs on all ABI stable platforms and verifies that an appropriate version floor is used for each, so running this test suite on multiple platforms is somewhat redundant.

Author: tshortli