[gn build] Make `ninja check-lld` also run LLD's unit tests

And add build files for gtest.

With this, the build files for //lld are complete.

Differential Revision: https://reviews.llvm.org/D55838


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349704 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed