[flang] Fix bug in tests for standalone build

When doing a standalone build of flang against an LLVM that contains a
built flang, the tests were run on the flang from LLVM rather than on
the one that was just built.

The problem was in the lit configuration for finding %flang etc.
Fix it to look only in the directory where it was built.

Differential Revision: https://reviews.llvm.org/D79327