Add codecoverage settings with reasonable tolerance

[rosesyrett]

Relates to bluesky/ophyd#1149

As the repository grows it's going to get more and more annoying seeing CI fail because of a 0.01% patch change.

So this is just to introduce some reasonable tolerances. In ophyd's case it was 0.1%, here it should probably be less (as there are far fewer tests here)

[prjemian]

Could you force the while clause to be called, only in CI, by adding a slow ophyd.Signal to the area detector? This would stabilize the code coverage.

"slow" would be enabled by subclassing ophyd.Signal and adding time.sleep(self.delay) before returning its value from .get().