Reverse "want" and "got" in test failures

[lutzky]

Suppose we have a rule configured to label messages with oops-bad-label, and a test that checks they're labeled with yep-correct-label. So before this change you would get:

--- want
+++ got
...
  "labels": [
-   "oops-bad-label"
+   "yep-correct-label"
  ]

To make this match the usual logic of automated testing (especially Go's "want" and "got"), we would like the "want" section to be "what the test describes should happen". Therefore, with this change, we get this output:

--- want
+++ got
...
  "labels": [
-   "yep-correct-label"
+   "oops-bad-label"
  ]

[mbrt]

Uhm, you're right! I never realized the test was the opposite.

[lutzky]

The failure (seen here) is unrelated to this commit... but it prevents tests from running. Would you like a separate PR to fix that?

[mbrt]

Ah yes, please. If you have a minute this would be helpful.

[lutzky]

I've done it as a separate commit within this PR, LMK if you want it in a separate PR.

[mbrt]

That's totally fine. Thanks a lot!