Fix a non-portable target reference in CMake helper function

[stmontgomery]

This fixes an oversight in a helper function in the project’s CMake rules: _swift_testing_install_target() has a hard-coded reference to the target named ”Testing” but it should instead refer to whatever target was passed in via the module argument to the function.

Discovered while working on #825.

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