test_pidfile_contents uses check_pid_process like it is a context manager, but it is not

[exarkun]

A recent CI run failed like this:

      | Traceback (most recent call last):
      |   File "/home/runner/work/tahoe-lafs/tahoe-lafs/.tox/pypy39/lib/pypy3.9/site-packages/allmydata/test/cli/test_run.py", line 196, in test_pidfile_contents
      |     with check_pid_process(pidfile):
      | AttributeError: 'NoneType' object is not a context manager (no __enter__/__exit__ method)
      | Falsifying example: test_pidfile_contents(
      |     content='0\r0',
      |     self=<allmydata.test.cli.test_run.RunTests.test_pidfile_contents id=0x4bfb088>,
      | )

Due to ​https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3983 the test managed to all the way to check_pid_process returning None instead of raising an exception and then the runtime got angry about with None:.