[perl #75486] Excessive resource use of `make spectest`

[Moritz Lenz via RT]

In edf00722595864310f52ab1f4eb757aff323e09f I've set the default number
of jobs to 1, and in a63c7e050fef6a648af234ac551e6b0d04db9fe4 I've moved
a long-running test to a separate stresstest target.