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

t/Makefile

@@ -35,6 +35,12 @@ all: $(DEFAULT_TEST_TARGET)
 test: pre-clean $(TEST_LINT)
 	$(MAKE) aggregate-results-and-cleanup
 
+failed:
+	@failed=$$(cd '$(TEST_RESULTS_DIRECTORY_SQ)' && \
+		grep -l '^failed [1-9]' *.counts | \
+		sed -n 's/\.counts$$/.sh/p') && \
+	test -z "$$failed" || $(MAKE) $$failed
+
 prove: pre-clean $(TEST_LINT)
 	@echo "*** prove ***"; $(PROVE) --exec '$(SHELL_PATH_SQ)' $(GIT_PROVE_OPTS) $(T) :: $(GIT_TEST_OPTS)
 	$(MAKE) clean-except-prove-cache