Ability to disable parallel test to spot non-termination test

[liufengyun]

It would be nice if we have the ability to disable parallel test to spot non-termination tests.

The obvious tweak of threadLimit doesn't help.

https://github.com/lampepfl/dotty/blob/0a2734bf7b4de534d449d86e3b196c46771eb754/compiler/test/dotty/tools/vulpix/ParallelTesting.scala#L911-L925

Interestingly, if threadLimit is set to < 2, the test doesn't run.

Edited: it's < 2 instead of < 3.