Make sure that post-test checks do not take up too much time

[mulkieran]

We are currently trying to ensure that any test that relies on the stratisd D-Bus signals and values waits 16 seconds after the last command is executed before it runs. We are about to add a second test that also needs to wait for the stratisd D-Bus layer to reach quiescence. We don't want two 16 second waits, because then the tests will take a much longer time. We don't want to build in some weird dependency between the two tests. We would like a shared counter so that each check knows how long it has been since the test was stopped already, and only uses as much as it needs. For that, we need a counter somewhere.

[mulkieran]

Closed by #242