Simplify iOS reporter API

[AbigailMcP]

At the moment, to use the report with XCTest you have to have a setup similar to this:

- (void)testBridge {
  // Make a new expectation
  XCTestExpectation *expectation = [[XCTestExpectation alloc] initWithDescription: @"Cavy tests passed"];
  
  [CavyNativeReporter onFinishWithBlock: ^void(NSDictionary* report) {
    long errorCount = [report[@"errorCount"] integerValue];
    if (errorCount > 0) {
      XCTFail(@"Cavy tests had one or more errors");
    }
    [expectation fulfill];
  }];
  
  [self waitForExpectations:@[expectation] timeout:60];
}

Fulfilling the expectation within the block passed to Cavy and waiting on those expectations with [self waitForExpectations:@[expectation] timeout:60];

It would be better to be able to do something like this:

- (void)testBridge {
  [CavyNativeReporter waitForReport:5];
  
  // Pull the error count from the report object.
  NSDictionary *report = CavyNativeReporter.cavyReport;
  long errorCount = [report[@"errorCount"] integerValue];
  
  XCTAssertEqual(errorCount, 0);
}

This might be achievable by adopting a similar technique to the Android module, making the report a public property on the class, and waiting on a Dispatch Group.

Would be great to achieve this without coupling the reporter to XCTest, but that might be tricky.