run-tests: print number of tests and parallel process count

Authored by indygreg.

Description

run-tests: print number of tests and parallel process count

This seems like a useful output message to have.

I also sneak in a change to lower the parallel process count if it
is larger than the number of tests, as that makes no sense and output
saying we're running more tests in parallel than there exists tests would
be wonky.

Differential Revision: https://phab.mercurial-scm.org/D5070