test-tap.at: Use PATH instead of hardcoding "$builddir/t"

test-tap.at: Use PATH instead of hardcoding "$builddir/t"

Hardcoding "$builddir/t" doesn't work at all if VPATH builds need to
run programs in "$srcdir", which we do now need.  Generally PATH
lookups are more flexible in Autotest and work out much better.

Technically this is a change in behaviour so some downstream projects
may need to be updated.