Testsuite: Fix issue with get_gpu_count with multiple GPU types

Create generic function get_gres_count to get the node counts of any
GRES, not just GPUs.
Make get_gpu_count able to parse any combination of GRES names and
types.
Create get_gpu_count wrapper of get_gres_count for backwards
compatibility.
Expand the regex to not include newlines.
Rename variable gpu_count to gres_count.

Bug 7995