Fix bug where non-GPU GRES lines were processed in GRES GPU plugin

Skip all GRES lines that aren't Name=gpu.
Write regression test to reproduce bug and test this patch.
Write regression tests to prove non-gpu GRESs are ignored.
Improve GRES GPU regression tests by running slurm.conf with
GresTypes=gpu,nic,mic,tmpdisk.
Document the regression tests some more.

Bug 6014