Explicitly set the number of threads as a tasty option instead of relying on environment variables

[michaelpj]

At the moment we have to set TASTY_NUM_THREADS to 1 otherwise some of the ghcide tests fail, but I think we can just set this in tasty itself using adjustOption. That would make things less error-prone for devs.

[bendo]

Will look at it.

[michaelpj]

Apparently it's awkward to do this correctly but here is a weird way to do it: UnkindPartition/tasty#406 (comment)

[dyniec]

I don't think mentioned solution works. But if we were to replace testGroup with sequentialTestGroup at every level of a tree we should get desired behavior.
EDIT: it works, I updated PR to use PlusTestOptions approach