Add Makefile support for yaml-test-suite

Add `test-suite` rule to Makefile.  This test clones the libyaml-test
repo (which then clones the libyaml-parser and libyaml-emitter repos)
and calls `make test` in it.  That in turn clone yaml/yaml-test suite.

Normally the subtests will clone the latest libyaml, but this test
passes the current libyaml via an environment variable.

Also add `test-all` to run this together with normal `make test` rule.

Makefile.am

@@ -12,9 +12,12 @@
 	-rm -f aclocal.m4 config.h.in configure config/*
 	-find ${builddir} -name Makefile.in -exec rm -f '{}' ';'
 
+distclean-local:
+	-rm -fr libyaml-test
+
 .PHONY: bootstrap
 bootstrap: maintainer-clean
 	./bootstrap
 
 test: all
 	make -C tests check-TESTS
@@ -15,6 +18,14 @@
 .PHONY: bootstrap
 bootstrap: maintainer-clean
 	./bootstrap
 
 test: all
 	make -C tests check-TESTS
+
+test-suite: libyaml-test
+	(export LIBYAML_DIR=$$PWD; make -C $< test)
+
+libyaml-test:
+	git clone https://github.com/yaml/$@
+
+test-all: test test-suite