[Flang][OpenMP] Move FIR lowering tests to a separate directory (#66779)

As part of preparing for the switch to HLFIR lowering, all OpenMP FIR
lowering tests are moved to a subdirectory in the OpenMP directory.
Copies of these tests that work with HLFIR will be created during this
week. After the switch the FIR lowering tests will be removed.

All new tests should be added with the HLFIR flow.