Parallel testing of multilibs

[Alexandre Oliva]

Tired of having to test dozens of multilibs one after another, I
decided it was time to be able to take advantage of `-j' to run tests
too.  Since I'm no tcl hacker to be able to fire up multiple tests in
parallel, I thought it would be easier and cleaner to let make take
care of that, just like it does for builds.  The trick was to create a
separate testsuite directory for each multilib variant, and let each
one run independent of the other.

If nobody protests vehemently, I'll check this in in the next 12-24
hours.