FutureClass.java

//In a class far far away...
@future
global static void RunMockCalloutForTest(String accountId){
  TestRestClient trc = new TestRestClient();
  id aId;
  try {
    aId = (Id) accountId;
  } catch (Exception e) {
    throw new exception('Failed to cast given accountId into an actual id, Send me a valid id or else.');
  }
  Account a = [select id, name, stuff, foo, bar from Account where id = :aId];
  
  //make your callout
  RestClientHTTPMocks fakeResponse = new RestClientHTTPMocks(200, 'Success', 'Success',  new Map<String,String>());
  System.AssertNotEquals(fakeResponse, null);
  Test.setMock(HttpCalloutMock.class, fakeResponse);
  System.AssertNotEquals(trc, null); //this is a lame assertion. I'm sure you can come up with something useful!
  String result = trc.get('http://www.google.com');
}