[api][test] typo `MockReponse` → `MockResponse`
[api][test] typo `MockReponse` → `MockResponse`

Fix typo in API unit tests: replace `MockReponse` with `MockResponse`.

clokep/datadogpy,KyleJamesWalker/datadogpy,KyleJamesWalker/datadogpy,clokep/datadogpy