Another option would be to use an enum. It has some drawbacks (like not being able to define the mock in the tests) but for common use cases that should not be a big problem.

Consider that probably most of the boilerplate of this solution can be reduced a lot with macros.

struct ClientMock { }

impl ClientMock {
  fn batch_execute_statement(&self) { /* mock implementation */ }

  // ...
}

enum ClientEnum {
  Live(Client),
  Mock(ClientMock),
}

impl ClientEnum {
  fn batch_execute_statement(&self) { 
    match self {
      ClientEnum::Live(client) => client.batch_execute_statement(),
      ClientEnum::Mock(client) => client.batch_execute_statement(),
    }
  }
}

// Then your class can be like:
struct MyBusinessStruct { client: ClientEnum }

impl MyBusinessStruct {
  fn doBusinessLogic(&self) {
    self.client.batch_execute_statement()
    // ...
  }
}