Use fixture (go-vcr) tests when available

[displague]

Very little is tested in this repository when a PR is created.
(make test BUILD=local: https://github.com/packethost/packngo/blob/master/Makefile#L34-L35)

With the go-vcr support that is currently present but disabled, we could start testing every function that has a recorded test and skip any test that doesn't have a recording. This will help us reach the goal of having fixtures for every test (and transition from disabled to play by default for PACKNGO_TEST_RECORDER).

I propose, when PACKNGO_TEST_ACTUAL_API=1 and PACKNGO_TEST_RECORDER=play and -test.short=1 is set, we ONLY skip tests without fixtures.

See https://github.com/packethost/packngo/blob/master/CONTRIBUTING.md#acceptance-tests for more context.