twisted.test.test_twistd tests are not safe to be run in parallel

[adiroiban]

To replicate, run

$ trial -j 2 -u twisted.test.test_twistd

===============================================================================
[ERROR]
Traceback (most recent call last):
  File "/home/adi/chevah/twisted/src/twisted/test/test_twistd.py", line 660, in test_applicationStartsWithConfiguredNameIDs
    self._applicationStartsWithConfiguredID(
  File "/home/adi/chevah/twisted/src/twisted/test/test_twistd.py", line 612, in _applicationStartsWithConfiguredID
    runner.preApplication()
  File "/home/adi/chevah/twisted/src/twisted/scripts/_twistd_unix.py", line 205, in preApplication
    checkPID(self.config["pidfile"])
  File "/home/adi/chevah/twisted/src/twisted/scripts/_twistd_unix.py", line 109, in checkPID
    sys.exit(
builtins.SystemExit: Another twistd server is running, PID 242523

This could either be a previously started instance of your application or a
different application entirely. To start a new one, either run it in some other
directory, or use the --pidfile and --logfile parameters to avoid clashes.


twisted.test.test_twistd.ApplicationRunnerTests.test_applicationStartsWithConfiguredNameIDs
===============================================================================
[ERROR]
Traceback (most recent call last):
  File "/home/adi/chevah/twisted/src/twisted/test/test_twistd.py", line 640, in test_applicationStartsWithConfiguredNumericIDs
    self._applicationStartsWithConfiguredID(
  File "/home/adi/chevah/twisted/src/twisted/test/test_twistd.py", line 612, in _applicationStartsWithConfiguredID
    runner.preApplication()
  File "/home/adi/chevah/twisted/src/twisted/scripts/_twistd_unix.py", line 205, in preApplication
    checkPID(self.config["pidfile"])
  File "/home/adi/chevah/twisted/src/twisted/scripts/_twistd_unix.py", line 109, in checkPID
    sys.exit(
builtins.SystemExit: Another twistd server is running, PID 242522

This could either be a previously started instance of your application or a
different application entirely. To start a new one, either run it in some other
directory, or use the --pidfile and --logfile parameters to avoid clashes.


twisted.test.test_twistd.ApplicationRunnerTests.test_applicationStartsWithConfiguredNumericIDs
-------------------------------------------------------------------------------
Ran 88 tests in 2.341s

FAILED (errors=2, successes=86)