test: Make a start on a test suite for libLTO. This works in a similar way to the gold plugin tests. We search for a compatible linker on $PATH and use it to run tests against our just-built libLTO. To start with, test the just added opt level functionality. Differential Revision: http://reviews.llvm.org/D8472 llvm-svn: 232785 GitOrigin-RevId: bbdc26ba1b05215347cae02242fb512586a9554e