[CMake] Use explicit RPATHs for unit test executables
Instead of letting CMake set RPATH of the test executables, use
INSTALL_RPATH and BUILD_WITH_INSTALL_RPATH just like other executable
and shared libraries.
Previously when a swift module link with exported swift-syntax targets e.g.
'SwiftSyntax::SwiftParser', the libraries in earlyswiftsyntax were used
instead of the copied libraries in the swift build directory. That
wasn't ideal.
