gh-101659: Isolate "obmalloc" State to Each Interpreter (gh-101660)

This is strictly about moving the "obmalloc" runtime state from
`_PyRuntimeState` to `PyInterpreterState`.  Doing so improves isolation
between interpreters, specifically most of the memory (incl. objects)
allocated for each interpreter's use.  This is important for a
per-interpreter GIL, but such isolation is valuable even without it.

FWIW, a per-interpreter obmalloc is the proverbial
canary-in-the-coalmine when it comes to the isolation of objects between
interpreters.  Any object that leaks (unintentionally) to another
interpreter is highly likely to cause a crash (on debug builds at
least).  That's a useful thing to know, relative to interpreter
isolation.

Lib/test/test_threading.py

@@ -1343,6 +1343,7 @@ class SubinterpThreadingTests(BaseTestCase):
             import test.support
             test.support.run_in_subinterp_with_config(
                 {subinterp_code!r},
+                use_main_obmalloc=True,
                 allow_fork=True,
                 allow_exec=True,
                 allow_threads={allowed},