Add elicit type when testing any method that returns a stdlib type

[tcharding]

The changes in #591 included

-    let _ = node.client.get_network_hash_ps().expect("rpc");
+    let _: f64 = node.client.get_network_hash_ps().expect("rpc");

We should do this for all methods that return stdlib types (i.e, returns boolean, numeric, or string)