unittests: Fix build with LLVM_LINK_LLVM_DYLIB=ON

The build system was linking the PluginsTests unittest against libLLVM.so
and LLVMAsmParser which was causing the test to fail with this error:

LLVM ERROR: inconsistency in registered CommandLine options

We need to add llvm libraries to LLVM_LINK_COMPONENTS so that
they are dropped from the linker arguments when linking with
LLVM_LINK_LLVM_DYLIB=ON

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D105523