[Cmake] Use Cmake's default RPATH for unittest.

Similarly to what was done in r280791 for llvm/.
This should fix a bunch of failures I saw while
trying a BUILD_SHARED_LIBS build on MacOS.
Still there are some failures, but this is a step
forward.

Thanks a lot to Chris Bieneman for the suggested
fix (in PR30345)

llvm-svn: 281256