Fix generics-related error in net/netip fuzz tests.

One of the helper functions requires generics to work correctly. Until
they are properly supported we have to stub it out.

Generics support is tracked in
gopherjs#1013.

compiler/natives/src/net/netip/fuzz_test.go

@@ -0,0 +1,10 @@
+//go:build js
+
+package netip_test
+
+import "testing"
+
+func checkStringParseRoundTrip(t *testing.T, x interface{}, parse interface{}) {
+	// TODO(nevkontakte): This function requires generics to function.
+	// Re-enable after https://github.com/gopherjs/gopherjs/issues/1013 is resolved.
+}