[tlbuild] make check done by make world

[Karl Berry]

The TeX Live Build script now also runs make check, which performs the
various tests which the subprograms are set up to run, mostly thanks to
Peter B.  There aren't too many of them, but any testing is all to the
good.

Please give it a whirl when you have a chance, so we can avoid
unpleasant surprises later.

In the event that it's impossible to get through, it can be ignored by
Build TL_MAKE_FLAGS=check_target=skip, or otherwise arranging for
passing check_target=skip to make.

Thanks,
Karl