make: Redirect unittest output to /dev/null

They are quite chatty and fail very rarely, so let's not have them spam our
logs. Failures should print to stderr anyway.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

Makefile

@@ -367,7 +367,7 @@ update-mocks/%: %
 	@tools/update-mocks.sh "$*"
 
 unittest/%: %
-	$(VALGRIND) $(VALGRIND_TEST_ARGS) $*
+	$(VALGRIND) $(VALGRIND_TEST_ARGS) $* > /dev/null
 
 # Installation directories
 prefix = /usr/local