Mock Out EPICS Calls in Tests

[eloise-nebula]

Is your feature request related to a problem? Please describe.
Our current tests will time out on EPICS calls every time, resulting in extremely slow testing.

Describe the solution you'd like
I'd like to mock out the EPICS calls to eliminate their burden in testing, or else figure out why our tests are running so slow.

Describe alternatives you've considered
Waiting 30 minutes per test run (not ideal).

[robnagler]

fyi https://github.com/slaclab/slicops/blob/main/slicops/mock_epics.py