run-tests: use go mod tidy

Use go mod tidy instead of go mod download in run-tests. The latter
changes go.sum for some reason.

See golang/go#43994

run-tests.sh

@@ -36,7 +36,7 @@ fetch() {
 
 # Pull all dependencies, so that we can run tests without the
 # vm having network access.
-go mod download
+go mod tidy
 
 # Use sudo if /dev/kvm isn't accessible by the current user.
 sudo=""