Issue #15168: Move importlb.test to test.test_importlib.

This should make the Linux distros happy as it is now easier to leave
importlib's tests out of their base Python distribution.

Lib/test/test_importlib/extension/util.py

@@ -0,0 +1,20 @@
+import imp
+from importlib import machinery
+import os
+import sys
+
+PATH = None
+EXT = None
+FILENAME = None
+NAME = '_testcapi'
+try:
+    for PATH in sys.path:
+        for EXT in machinery.EXTENSION_SUFFIXES:
+            FILENAME = NAME + EXT
+            FILEPATH = os.path.join(PATH, FILENAME)
+            if os.path.exists(os.path.join(PATH, FILENAME)):
+                raise StopIteration
+    else:
+        PATH = EXT = FILENAME = FILEPATH = None
+except StopIteration:
+    pass