[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.

Reviewers: gbreynoo

Reviewed By: gbreynoo

Subscribers: llvm-commits, bkramer

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62930

llvm-svn: 363189