`workspace/tests` blocking other requests from executing

[ahoppen]

#1724 identified that no requests are being handled while workspace/tests is running, effectively blocking all LSP-based functionality while VS Code is getting the list of tests. Figure out whether we can relax the dependency constraints between requests here to allow requests to run concurrently to workspace/tests.

[ahoppen]

Synced to Apple’s issue tracker as rdar://137886460

[ahoppen]

As far as I can tell, the issue was a $/setTrace notification after the workspace/tests request, which was blocking text document requests requests from executing. #1871 should have fixed that.