[flang] Test cleanup

Fix omp-nested01.f90 so that it is not an expected failure. The test was
never enabled but I'm guessing this is what it's supposed to do.

Fix the instructions to include "make test" as part of running tests.

Original-commit: flang-compiler/f18@c351181cabb234f73cb09c23e45dfe0bfc2d7219
Reviewed-on: https://github.com/flang-compiler/f18/pull/1075