Issue #14637: Fix the UNC import test under Windows to actually use

the UNC path. Also clean up sys.path and invalidate finder caches.

Thanks to Vinay Sajip for spotting the use of the wrong path.

Lib/test/test_import.py

@@ -459,6 +459,7 @@ class PathsTests(unittest.TestCase):
     def test_UNC_path(self):
         with open(os.path.join(self.path, 'test_trailing_slash.py'), 'w') as f:
             f.write("testdata = 'test_trailing_slash'")
+        importlib.invalidate_caches()
         # Create the UNC path, like \\myhost\c$\foo\bar.
         path = os.path.abspath(self.path)
         import socket
@@ -466,10 +467,13 @@ class PathsTests(unittest.TestCase):
         drive = path[0]
         unc = "\\\\%s\\%s$"%(hn, drive)
         unc += path[2:]
-        sys.path.append(path)
+        sys.path.append(unc)
+        try:
             mod = __import__("test_trailing_slash")
             self.assertEqual(mod.testdata, 'test_trailing_slash')
             unload("test_trailing_slash")
+        finally:
+            sys.path.remove(unc)
 
 
 class RelativeImportTests(unittest.TestCase):