unittests: Don't install TestPlugin.so

Summary:
add_llvm_loadable_module adds an install target by default, but this
module is only used for a unit test, so we don't need to install it.

Reviewers: philip.pfaffe, thakis

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D50668

llvm-svn: 339897