create the tests directory before the automake generated wrapper races for it

am__check_pre in the automake-generated Makefile basically runs

	test -d tests || mkdir tests || exit $?

When running out-of-tree with parallel make (as vtest does), this
becomes racy. Avoid the race by a mkdir ignoring the error

bin/varnishtest/Makefile.am

@@ -9,6 +9,7 @@ check: check-am check-local
 check-am: check-local
 # See if list of checks have changed, recheck
 check-local:
+	mkdir tests 2>/dev/null || true ; \
 	LC_ALL=C; \
 	if [ "$$(cd $(srcdir) && echo tests/*.vtc)" != "@VTC_TESTS@" ]; then \
 		cd $(top_builddir) && ./config.status --recheck ; \