test_pidfile_contents sometimes fails with ProcessInTheWay

[exarkun]

On a recent CI run it 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):
      |   File "/home/runner/work/tahoe-lafs/tahoe-lafs/.tox/pypy39/lib/pypy3.9/site-packages/allmydata/util/pid.py", line 82, in check_pid_process
      |     raise ProcessInTheWay(
      | allmydata.util.pid.ProcessInTheWay: A process is already running as PID 1
      | Falsifying example: test_pidfile_contents(
      |     content='1\r0',
      |     self=<allmydata.test.cli.test_run.RunTests.test_pidfile_contents id=0x4bfb088>,
      | )

Notice that content was generated as "1\r0". It looks like the "good content" regex fails to match this but the parser in the implementation still manages to pull out the "1" and "0" and tries to operate on them.