Fix typo s/propogate/propagate/.
Use constants in configOriginIsEnabled()

To indicate that they are test parameters and immutable whilst the test is
running. We've done this with a couple of the recently added tests and it
matches how we use `let()` in rspec.

I've also taken the opportunity to replace `time.ParseDuration()` with
`time.Duration()` because:

- We can't discard the error when assigning a constant (at least I don't
  think we can).
- There's less chance that we're going to typo the value and not notice.
- I actually think it reads a little nicer.
Rename function that verifies edge is healthy

Because we're testing that the edge thinks origin is healthy, rather than
verifying that origin itself is healthy. Amend the log message to match. Add
a bit of documentation that explains why we have to do this.
Merge pull request #13 from alphagov/minor_changes_probe_helper

Minor changes probe helper