Merge branch 'js/re-running-failed-tests'

"make -C t failed" will now run only the tests that failed in the
previous run.  This is usable only when prove is not use, and gives
a useless error message when run after "make clean", but otherwise
is serviceable.

* js/re-running-failed-tests:
  t/Makefile: add a rule to re-run previously-failed tests