Avoid shadowed module filename in dynamic metadata fixture

The fixture left a stale __file__ value beneath a module subclass property.
LOAD_ATTR_MODULE specialization can read that value, making the assertion
depend on warmup state in the existing filename fallback, including main.

Remove the shadow value and exercise warmed getfile access, two changing
origins, and independent cold frame/traceback lookups. Keep production code
unchanged.

Verified the corrected test 50 times on each of main and the candidate;
all six focused suites pass (1,135 tests reported, four skipped).

Author: mdeck

Lib/test/test_inspect/test_inspect.py

@@ -1155,11 +1155,22 @@ class DynamicModule(types.ModuleType):
             @property
             def __file__(self):
                 return self.source
+        # Do not leave a shadow value that LOAD_ATTR_MODULE can read instead
+        # of the property when attribute access specializes.
+        del self.module.__dict__['__file__']
         self.module.__class__ = DynamicModule
-        self.module.source = self.filename + '.dynamic'
-        frame, traceback = self.capture(filename=self.module.source)
-        for object in (frame, traceback):
-            self.assertIs(inspect.getmodule(object), self.module)
+        for suffix in ('.dynamic', '.changed'):
+            with self.subTest(suffix=suffix):
+                self.module.source = self.filename + suffix
+                frame, traceback = self.capture(filename=self.module.source)
+                # Exercise warmed attribute access as well as cold caches.
+                for _ in range(100):
+                    self.assertEqual(inspect.getfile(self.module),
+                                     self.module.source)
+                for object in (frame, traceback):
+                    inspect.modulesbyfile.clear()
+                    inspect._filesbymodname.clear()
+                    self.assertIs(inspect.getmodule(object), self.module)
 
     def test_sourceless_import(self):
         import py_compile