… after `Stop`

Retaining files after `Stop` can help debug some test-failures because
 one can start the server back up (off the store-dir left behind after
 failure). Looking at the data the failing test left behind can help
 hypothesize better.

…leCleanup`

`BasedirOverride` allows user to set testserver base-dir (as opposed to
 using a random-dir generated by testserver lib. This helps test
 `NoFileCleanup` better as preserved store-dir's state can now be asserted
 upon after original testserver's teardown.