tests: Use $(MKDIR_P) to avoid races.

"test -d x || mkdir x" has a race when invoked in parallel: it is possible
for two processes to both see that 'x' does not exist and both try to
create it, and if that happens then one of them will fail.  This avoids
the problem.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>

tests/automake.mk

@@ -207,7 +207,7 @@ valgrind_wrappers = \
 	tests/valgrind/test-vconn
 
 $(valgrind_wrappers): tests/valgrind-wrapper.in
-	@test -d tests/valgrind || mkdir tests/valgrind
+	@$(MKDIR_P) tests/valgrind
 	$(AM_V_GEN) sed -e 's,[@]wrap_program[@],$@,' \
 		$(top_srcdir)/tests/valgrind-wrapper.in > $@.tmp && \
 	chmod +x $@.tmp && \