gh-152548: Skip isolated() tests without subprocess support

@isolated() always runs the test in a subprocess, so skip it in the
parent process on platforms that lack subprocess support, the same way
the tests it replaces were guarded by requires_subprocess().

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Doc/library/test.rst

@@ -996,6 +996,8 @@ The :mod:`!test.support` module defines the following functions:
    :func:`requires_resource`, :func:`requires`, :func:`bigmemtest` and the like
    behave consistently in both processes.
 
+   The test is skipped on platforms without subprocess support.
+
 
 .. data:: running_isolated
 

Lib/test/support/_isolation.py

@@ -81,6 +81,14 @@ def _remote(detail):
     return _RemoteTraceback('\n"""\n%s"""' % detail)
 
 
+def _check_subprocess_support():
+    # isolated() always runs the test in a subprocess, so skip (in the parent)
+    # on platforms that do not support spawning one.
+    import test.support as support
+    if not support.has_subprocess_support:
+        raise unittest.SkipTest('requires subprocess support')
+
+
 def _run_in_subprocess(module, qualname):
     """Run module.qualname (a test method or class) in a fresh subprocess.
 
@@ -155,6 +163,7 @@ def wrapper(self, /, *args, **kwargs):
         if running_isolated:
             # Already running in the subprocess: run the real test.
             return func(self, *args, **kwargs)
+        _check_subprocess_support()
         cls = type(self)
         qualname = f'{cls.__qualname__}.{func.__name__}'
         outcomes, output, returncode = _run_in_subprocess(cls.__module__,
@@ -177,6 +186,7 @@ def setUpClass(cls):
         if running_isolated:
             orig_setUpClass(cls)
             return
+        _check_subprocess_support()
         # Run the whole class in a single subprocess and stash the outcomes
         # for the wrapped test methods to replay.
         outcomes, output, returncode = _run_in_subprocess(cls.__module__,
@@ -243,6 +253,8 @@ def isolated():
     skipped are reported individually.  The original subprocess traceback is
     shown as the cause of a reported failure or error.  Use
     :data:`running_isolated` in fixtures to choose what to run in the subprocess.
+
+    The test is skipped on platforms without subprocess support.
     """
     def decorator(obj):
         if isinstance(obj, type):