Prior to this commit, the required runtime dependencies were checked
via reflection each time an attempt was made to instantiate
MicrometerObservationRegistryTestExecutionListener.
Since it's sufficient to check for the presence of required runtime
dependencies only once, this commit caches the results of the
dependency checks in a static field.
This commit also introduces automated tests for the runtime dependency
checks in MicrometerObservationRegistryTestExecutionListener.
See gh-30747
5aac35b99e