fix: force clean test results directory

Without `-f`, `make clean` fails on machines that don't have the dir.

cc @jbenet

test/Makefile

@@ -14,7 +14,7 @@ all: clean deps $(T) aggregate
 
 clean:
 	@echo "*** $@ ***"
-	-rm -r test-results
+	-rm -rf test-results
 
 $(T):
 	@echo "*** $@ ***"