gh-76785: Fixes for test.support.interpreters (gh-112982)

This involves a number of changes for PEP 734.

Lib/test/test_interpreters/utils.py

@@ -0,0 +1,73 @@
+import contextlib
+import os
+import threading
+from textwrap import dedent
+import unittest
+
+from test.support import interpreters
+
+
+def _captured_script(script):
+    r, w = os.pipe()
+    indented = script.replace('\n', '\n                ')
+    wrapped = dedent(f"""
+        import contextlib
+        with open({w}, 'w', encoding='utf-8') as spipe:
+            with contextlib.redirect_stdout(spipe):
+                {indented}
+        """)
+    return wrapped, open(r, encoding='utf-8')
+
+
+def clean_up_interpreters():
+    for interp in interpreters.list_all():
+        if interp.id == 0:  # main
+            continue
+        try:
+            interp.close()
+        except RuntimeError:
+            pass  # already destroyed
+
+
+def _run_output(interp, request, channels=None):
+    script, rpipe = _captured_script(request)
+    with rpipe:
+        interp.exec_sync(script, channels=channels)
+        return rpipe.read()
+
+
+@contextlib.contextmanager
+def _running(interp):
+    r, w = os.pipe()
+    def run():
+        interp.exec_sync(dedent(f"""
+            # wait for "signal"
+            with open({r}) as rpipe:
+                rpipe.read()
+            """))
+
+    t = threading.Thread(target=run)
+    t.start()
+
+    yield
+
+    with open(w, 'w') as spipe:
+        spipe.write('done')
+    t.join()
+
+
+class TestBase(unittest.TestCase):
+
+    def pipe(self):
+        def ensure_closed(fd):
+            try:
+                os.close(fd)
+            except OSError:
+                pass
+        r, w = os.pipe()
+        self.addCleanup(lambda: ensure_closed(r))
+        self.addCleanup(lambda: ensure_closed(w))
+        return r, w
+
+    def tearDown(self):
+        clean_up_interpreters()