Centralize LD_LIBRARY_PATH logic with new SET_TEST_PATH macro.

Add ${SHARED_LIB_STAGING_DIR_RELEASE} to LD_LIBRARY_PATH for executing tests:
otherwise Debug-build tests can't find (e.g.) the aprutil DLL, for which we
don't build/package a debug variant.
Leverage discovery that a CMake macro can accept a target variable name
argument for LL_TEST_COMMAND macro.