Use the '-define-availability' frontend flag directly in CMake rules

[stmontgomery]

This replaces usages of the -enable-experimental-feature AvailabilityMacro=X Swift frontend flag in swift-testing's CMake rules with -define-availability X.

Motivation:

The former flags are used in Package.swift as a workaround because SwiftPM does not allow using unsafe flags in package dependencies. But this workaround is not necessary when building with CMake—we can use the -define-availability flag directly without restriction.

Using this workaround when building via CMake with Library Evolution enabled causes every specified availability macro flag to be printed in the textual .swiftinterface file for the module. This information is not needed by clients of the Testing module, so the primary motivation of this change is to remove those unnecessary flags from that interface file.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[stmontgomery]

@swift-ci please test