Add best-effort scenario to clear shared environment

[BenjaminPelletier]

Is your feature request related to a problem? Please describe.
Currently, the F3548-21 test suite allows specification of flight planners that are present in the environment, but not part of the test -- this enables clearing of the shared environment of potentially-lingering resources from those flight planners not otherwise being tested. However, because the prep planners test scenario is used for this purpose, the test run will fail if these otherwise-uninvolved flight planners cannot be successfully instructed to clear the area. This is problematic when these uninvolved participants may not have an expectation of persistent availability, and therefore may reasonably be unavailable when attempting to run a test in which they are not directly involved.

Describe the solution you'd like
Update the prep planners test scenario to accept both flight planners under test and flight planners in the environment. All flight planners under test should continue to have the same failed checks when they do not successfully clear the area. All flight planners in the environment but not in flight planners under test should be issued the same kind of requests to clear the area, but failure of those requests should result merely in a note and not a test check failure.

Describe alternatives you've considered
There are other potential approaches such as a separate scenario, but adjusting the behavior of this existing scenario in the described way seems the most clear and straightforward at the time of writing this issue.

Additional context
Relevant to US UTM Implementation usage of automated testing.