getFromRestAPI: test each_record, not getFromRestAPI

refs #6964
FPRClient: Mark getFromRestAPI function as private

Outside code should prefer using each_record() now.
Also rename to pothole_case.

refs #6964
FPRClient: Replace fprserver fixture with constant

Since this was unchanging, it can simply be a constant string without
breaking anything.

Since pytest fixtures don't always play nicely with VCR, it's easier
to sidestep this issue where possible.

refs #6964
FPRClient: update tests

* Add new test for each_record
* Adjust both each_record and getFromRestAPI tests to use VCR, and
  add recorded responses for both

refs #6964