[llvm-ar][test] Relax lit directory assumptions in thin-archive.test
Summary: thin-archive.test assumes the Output/<testname> structure that lit creates. Rewrite the test in a way that still tests the same thing (creating via relative path and adding via absolute path) but doesn't assume this specific lit structure, making it possible to run in a lit emulator.
Reviewed By: gbreynoo
Subscribers: llvm-commits, bkramer
Differential Revision: https://reviews.llvm.org/D62930