Don't install clang-import-test

I have been trying to reduce the installed size of our CHERI toolchain and
noticed that this tool was being installed even with -DLLVM_INSTALL_TOOLCHAIN_ONLY=ON.
This appears to be a test binary that should not be installed.

Reviewed By: v.g.vassilev
Differential Revision: https://reviews.llvm.org/D82169