support/testing/run-tests: unbreak on Debian testing/unstable

Commit 3d2141bcee("support/testing/run-tests: specify multiprocessing
method") added a call to multiprocessing.set_start_method('fork') as a
workaround for python 3.14, which changed the default start method to
forkserver - Which is incompatible with the nose2 setup.

multiprocessing.set_start_method() is only supposed to be called a maximum
of 1 time per process and throws a RuntimeError if called more than that
(even with the same arguments):

>>> import multiprocessing
>>> multiprocessing.set_start_method('fork')
>>> multiprocessing.set_start_method('fork')
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    multiprocessing.set_start_method('fork')
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "/usr/lib/python3.13/multiprocessing/context.py", line 247, in set_start_method
    raise RuntimeError('context has already been set')

Debian included a similar patch in python3-nose2 0.51.1-2 (currently in
testing/unstable) which adds its own call to set_start_method():

https://salsa.debian.org/python-team/packages/nose2/-/blob/debian/0.15.1-2/debian/patches/0004-plugins-mp-set-context-to-fork-for-Python-3.14-mp-AP.patch?ref_type=tags

Which comes from:
https://github.com/nose-devs/nose2/pull/644

As discussed in the upstream PR, this is not a correct fix is wrong and
breaks various use cases.  An issue has been opened to get this fixed in the
Debian packaging at:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1129350

But until that is done, rework the patch to:

- Only override set_start_method() if needed to limit impact
- Monkey patch set_start_method() so additional calls are ignored

To unbreak run-test on affected Debian systems and add some documentation to
make it clear why this is done.

[Peter: use allow_none / force optional arguments as pointed out by Julien]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/testing/run-tests

@@ -142,5 +142,15 @@ def main():
 
 
 if __name__ == "__main__":
-    multiprocessing.set_start_method("fork")
+    # python 3.14 changed default start method from fork to
+    # fork-server, which is not compatible with the nose2 setup
+    if multiprocessing.get_start_method(allow_none=True) != "fork":
+        multiprocessing.set_start_method("fork", force=True)
+        # set_start_method throws a RuntimeError if called more than
+        # once.
+        # Debian python3-nose2 0.15.1-2 includes a patch adding
+        # another set_start_method() call, so monkey patch it out to
+        # get rid of this
+        multiprocessing.set_start_method = lambda *args: None
+
     sys.exit(main())