Explicitly disable Go module support for testing with Go >= 1.15

diff --git a/Makefile b/Makefile
index e072c1b..f301f00 100644
--- a/Makefile
+++ b/Makefile
@@ -83,8 +83,8 @@ format: earlyoom
 
 test: earlyoom
 	cppcheck -q . || echo "skipping optional cppcheck"
-	go test -v
+	GO111MODULE=off go test -v
 
 .PHONY: bench
 bench:
-	go test -run=NONE -bench=.
+	GO111MODULE=off go test -run=NONE -bench=.