tests: don't run test-update-atomic.t on chg

I am unable to find a good way to make `showwrites` extension in it to work with
chg. Also putting the use of showwrites inside `if no-chg` will defeat the
purpose of test, so I just made the test no-op on chg.

tests/test-update-atomic.t

@@ -1,4 +1,4 b''
-#require execbit unix-permissions
+#require execbit unix-permissions no-chg
 
 Checking that experimental.atomic-file works.