Make the testsuite run on machines without valgrind

tests/run

@@ -1,3 +1,9 @@
 #!/bin/sh
 
-cat $@ | valgrind --error-exitcode=1 -q --leak-check=full ./jq --run-tests
+if which valgrind > /dev/null; then
+    VALGRIND='valgrind --error-exitcode=1 -q --leak-check=full'
+else
+    VALGRIND=
+fi
+
+cat $@ | $VALGRIND ./jq --run-tests