Is your feature request related to a problem? Please describe.
QL tests in the Test Explorer occasionally go into the "retired" state, where the test result icon is faded. There seems to be little rhyme or reason to when this happens.

Describe the solution you'd like
We should properly implement the retire event on the Test Adapter. It was added to Test Explorer after our original implementation, so it seems to use the default behavior of retiring all tests after any reload of the test tree, which we probably do whenever any file in a test directory changes.

Additional context
See here for some basic information about how the retire event is supposed to work.