[test] Propagate CMake shared linker flags for tests to use (#85623)
The same that some setups needs the `CMAKE_EXE_LINKER_FLAGS` propagated
for the test to compile plugins in the same configuration as the
compiler was using, the `CMAKE_SHARED_LINKER_FLAGS` needs to be
propagated if one wants to match the configuration used by CMake to
create libraries during the tests.
Add a new `config` value derived from `CMAKE_EXE_LINKER_FLAGS`, turn
that value into a substitution for the `Macros` tests, and use the
substibution in one test that creates a shared object. This allows the
shared object to be linked in the same way that it has been configured
in CMake.
swift
83a9913a - [test] Propagate CMake shared linker flags for tests to use (#85623)
