Make missing pynvml an error for GPU tests

[bryevdv]

This PR makes a small change to the test runner to make pynvml required for stages that access GPU information. Previously, if pynvml was missing, and the GPU stage was run, system.gpus would return an empty list of GPU specs, resulting in a cryptic error suggesting that there were no GPUs present (even if there were):

Traceback (most recent call last):
  File "./test.py", line 30, in <module>
    plan = TestPlan(config, system)
  File "/home/idemeshko/development/cunumeric_clean/tests/_utils/test_plan.py", line 45, in __init__
    self._stages = [
  File "/home/idemeshko/development/cunumeric_clean/tests/_utils/test_plan.py", line 46, in <listcomp>
    STAGES[feature](config, system) for feature in config.features
  File "/home/idemeshko/development/cunumeric_clean/tests/_utils/test_stages/gpu.py", line 43, in __init__
    self.workers = self.compute_workers(config, system)
  File "/home/idemeshko/development/cunumeric_clean/tests/_utils/test_stages/gpu.py", line 66, in compute_workers
    assert len(gpus)
AssertionError

With this change, a GPU stage will instead fail with a clear import error:

dev38 ❯ ./test.py --use=cuda                                  
Traceback (most recent call last):
  File "./test.py", line 30, in <module>
    plan = TestPlan(config, system)
  File "/home/bryan/work/cunumeric/tests/_utils/test_plan.py", line 45, in __init__
    self._stages = [
  File "/home/bryan/work/cunumeric/tests/_utils/test_plan.py", line 46, in <listcomp>
    STAGES[feature](config, system) for feature in config.features
  File "/home/bryan/work/cunumeric/tests/_utils/test_stages/gpu.py", line 43, in __init__
    self.workers = self.compute_workers(config, system)
  File "/home/bryan/work/cunumeric/tests/_utils/test_stages/gpu.py", line 64, in compute_workers
    gpus = system.gpus
  File "/home/bryan/work/cunumeric/tests/_utils/system.py", line 102, in gpus
    import pynvml  # type: ignore[import]
ModuleNotFoundError: No module named 'pynvml'

Non-GPU stages e.g "eager" or "cpus" are unaffected and continue to run with or without pynvml installed.

[ipdemes]

LGTM