[3.13] gh-154272: Skip more forkserver tests if the start method is unavailable (GH-154499) (GH-154507)

Two tests were still not skipped: they were only guarded against Cygwin
and Windows.

(cherry picked from commit a2a8466)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/test/test_concurrent_futures/test_init.py

@@ -1,5 +1,6 @@
 import contextlib
 import logging
+import multiprocessing
 import queue
 import time
 import unittest
@@ -140,6 +141,8 @@ def test_spawn(self):
         self._test(ProcessPoolSpawnFailingInitializerTest)
 
     @support.skip_if_sanitizer("TSAN doesn't support threads after fork", thread=True)
+    @unittest.skipUnless("forkserver" in multiprocessing.get_all_start_methods(),
+                         "forkserver start method is not available")
     def test_forkserver(self):
         self._test(ProcessPoolForkserverFailingInitializerTest)