Fix Driver tests to check return value of parse(), simplify subclassing, and remove unneeded instance variables

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@186440 91177308-0d34-0410-b5e6-96231b3b80d8
build lld unittests as part of lld-test target

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@186441 91177308-0d34-0410-b5e6-96231b3b80d8