Fix calls to assert_called_once in unit tests

Mock has a method called assert_called_once_with to check that a mock
was called and the arguments it took were as expected. Mock does not
have a method called assert_called_once and calling it just creates a
mock bound to that name. This means that not only is nothing tested
when assert_called_once is used, the tests also don't warn about this.

This commit attempts to address this in two ways:
    - all occurrences of assert_called_once are replaced with a real
      assertion.
    - the hacking check that nova uses to guard against this has been
      copied to cinder's local hacking checks.

Fixing the assert_called_once issues also highlighted other mistakes
in certain tests which were addressed to make the tests pass.

Due to the nature of mock, this issue is also possible if a method is
misspelt or just mistakenly used and so the hacking check is only
addressing one very specific case. That said, it does appear to be a
common mistake and so is worth singling out.

Change-Id: Iedcc3f48d91f7ebd8878ccc3bca3d023503774bd
Closes-Bug: #1394544

cinder/hacking/checks.py

@@ -113,9 +113,21 @@ def check_no_log_audit(logical_line):
         yield(0, "N324: Found LOG.audit.  Use LOG.info instead.")
 
 
+def check_assert_called_once(logical_line, filename):
+    msg = ("N327: assert_called_once is a no-op. please use assert_called_"
+           "once_with to test with explicit parameters or an assertEqual with"
+           " call_count.")
+
+    if 'cinder/tests/' in filename:
+        pos = logical_line.find('.assert_called_once(')
+        if pos != -1:
+            yield (pos, msg)
+
+
 def factory(register):
     register(no_vi_headers)
     register(no_translate_debug_logs)
     register(no_mutable_default_args)
     register(check_explicit_underscore_import)
     register(check_no_log_audit)
+    register(check_assert_called_once)