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/geth/client/rpc_client.go

@@ -0,0 +1,30 @@
+package client
+
+import (
+	"context"
+	"github.com/ethereum/go-ethereum/rpc"
+)
+
+type RpcClient struct {
+	client  *rpc.Client
+	ipcPath string
+}
+
+func NewRpcClient(client *rpc.Client, ipcPath string) RpcClient {
+	return RpcClient{
+		client:  client,
+		ipcPath: ipcPath,
+	}
+}
+
+func (client RpcClient) CallContext(ctx context.Context, result interface{}, method string, args ...interface{}) error {
+	return client.client.CallContext(ctx, result, method, args)
+}
+
+func (client RpcClient) IpcPath() string {
+	return client.ipcPath
+}
+
+func (client RpcClient) SupportedModules() (map[string]string, error) {
+	return client.client.SupportedModules()
+}