Add test to prove WireMockRule as @Rule resets server between tests

Hi,

Just adding a quick additional test to prove that WireMockRule, when run as a Rule, resets the WireMock server between tests. This was already proven with WireMockClassRule (with the Rule/ClassRule combo).

I've also fixed a typo in a helper method name (and extracted a couple of helper methods to be shared across tests).

Thanks,
Rowan