Consolidate test doubles

- Migrate various mocks of core namespaces to shared version in `fakes` pkg
- Err on the side of making test doubles less sophisticated
- Don't pull over mocks of namespaces that are only used in example code

pkg/core/rpc_client.go

@@ -0,0 +1,9 @@
+package core
+
+import "context"
+
+type RpcClient interface {
+	CallContext(ctx context.Context, result interface{}, method string, args ...interface{}) error
+	IpcPath() string
+	SupportedModules() (map[string]string, error)
+}