Update vscode/test-electron to 2.3.8, fixing CI issues

[zgrannan]

Our CI seems to be failing due to microsoft/vscode-test#246; updating vscode/test-electron to the latest version should fix it.

[zgrannan]

The tests are still failing, but now for a different reason.

Some output from the macos tests:

Error: The following processes have been found: Process found: '6275 z3
6276 z3
6277 z3
6278 z3
6280 z3

[rayman2000]

So the other error occurs due to processes not being cleaned up properly on Windows and Mac sometimes. I am trying to find a solution to that, but I think we can merge this right away, the problems are entirely unrelated as far as I can tell.