gn build: Add a missing dependency from llvm/test to llvm-lit

gn build: Add a missing dependency from llvm/test to llvm-lit

check-llvm already listed llvm-lit as script which counts as a dep, so running
check-llvm worked fine, but `ninja -C out/gn llvm/test` didn't build llvm-lit
before if it wasn't already there.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@352893 91177308-0d34-0410-b5e6-96231b3b80d8