[3.13] gh-154272: Skip forkserver tests when the start method is unavailable (GH-154274) (GH-154472)

(cherry picked from commit 13e3f8a)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/test/test_concurrent_futures/util.py

@@ -130,6 +130,8 @@ def get_context(self):
             self.skipTest("require unix system")
         if support.check_sanitizer(thread=True):
             self.skipTest("TSAN doesn't support threads after fork")
+        if "forkserver" not in multiprocessing.get_all_start_methods():
+            self.skipTest("forkserver start method is not available")
         return super().get_context()
 
     def create_event(self):

Lib/test/test_multiprocessing_forkserver/__init__.py

@@ -1,3 +1,4 @@
+import multiprocessing
 import os.path
 import sys
 import unittest
@@ -9,5 +10,10 @@
 if sys.platform == "win32":
     raise unittest.SkipTest("forkserver is not available on Windows")
 
+# The forkserver start method requires passing file descriptors over a Unix
+# socket, which is not available on every platform (e.g. Solaris/illumos).
+if "forkserver" not in multiprocessing.get_all_start_methods():
+    raise unittest.SkipTest("forkserver start method is not available")
+
 def load_tests(*args):
     return support.load_package_tests(os.path.dirname(__file__), *args)