[3.15] gh-154272: Skip more forkserver tests if the start method is unavailable (GH-154499) (GH-154505)

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
@@ -147,6 +148,8 @@ 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)
 

Lib/test/test_profiling/test_tracing_profiler.py

@@ -1,4 +1,5 @@
 """Test suite for the cProfile module."""
+import multiprocessing
 import sys
 import unittest
 
@@ -220,8 +221,8 @@ def test_process_spawn_pickle(self):
         # gh-140729: test use Process in cProfile.
         self._test_process_run_pickle('spawn')
 
-    @unittest.skipIf(sys.platform == 'win32',
-                     "No 'forkserver' start method on Windows")
+    @unittest.skipUnless("forkserver" in multiprocessing.get_all_start_methods(),
+                         "forkserver start method is not available")
     def test_process_forkserver_pickle(self):
         # gh-140729: test use Process in cProfile.
         self._test_process_run_pickle('forkserver')