running make check with only specified tests

I've often wanted to be able to run "make check" and just have it run
the small number of tests I am interested in. Here's a tiny patch along
those lines. It creates a new targe which I have called "check-with" for
want of a better name. And with it I can do:

    $ make check-with TESTS="json jsonb"


and have it do the temp install etc and then run just those two tests.

Thoughts?

cheers

andrew