Revert "gh-128639: Don't assume one thread in subinterpreter finalization (gh-128640)" (gh-134256)

This reverts commit 9859791f9e.

The original change broke the iOS and android buildbots, where the tests are run single-process.

Programs/_testembed.c

@@ -1395,12 +1395,9 @@ static int test_audit_subinterpreter(void)
     PySys_AddAuditHook(_audit_subinterpreter_hook, NULL);
     _testembed_initialize();
 
-    PyThreadState *tstate = PyThreadState_Get();
-    for (int i = 0; i < 3; ++i)
-    {
-        Py_EndInterpreter(Py_NewInterpreter());
-        PyThreadState_Swap(tstate);
-    }
+    Py_NewInterpreter();
+    Py_NewInterpreter();
+    Py_NewInterpreter();
 
     Py_Finalize();