Stale nodes stay in test explorer

[eleanorjboyd]

If you don't do a full reload, stale test nodes will exist in the test explorer.

[eleanorjboyd]

The problem is demonstrated here. I switched from a different start directory (and maybe from unittest to pytest) and the node for blank-folder exists still even though I switched and reran discovery with the root as the start directory and therefore it rendered another node for the head.