Reduce UI test congestion

**Is your feature request related to a problem? Please describe.**
Looking at dumps of test runs (e.g. during #3358) or breaking test runs in a debugger it appears there are dozens of test threads stuck starting up or winding down.

Generally there seems to be a lot of congestion on the WinForms side.

I don't know if reducing congestion makes test runs complete faster, but it certainly will make inspecting hangs and crashes easier if there are less threads to look at.

**Describe the solution you'd like and alternatives you've considered**
- consider limiting the amount of WinFormsFact/Theory threads active at a time
- consider investigating and reducing congestion when running multiple UI threads

**Will this feature affect UI controls?**

no, this affects infrastructure running multiple UI threads, primary use-case being unit tests

/cc @AArnott @hughbe @RussKie - note this is not of high priority, just want to ping you if you weren't aware that there are often several dozens of threads waiting on a global lock for startup/shutdown