fix(fakeAsyncTest): fix #937, let user be able to customize testable macroTask

[JiaLiPassion]

fix #937
let user be able to customize which macroTasks can be test with fakeAsyncTestSpec.
for example, in #937, user want to test cavans.toBlob(). with this PR, user can achieve it in two ways.

1. pass the options from FakeAsyncTestSpec constructor

const testZoneSpec = new FakeAsyncTestZoneSpec('name', false, [{source: 'HTMLCanvasElement.toBlob'}]);

2. sometimes application test code can't access FakeAsyncTestSpec initialize process, so user can also define a global variable before load fakeAsyncTest.

• browser

 window['__zone_symbol__FakeAsyncTestMacroTask'] = [{
     source: 'HTMLCanvasElement.toBlob',
     callbackArgs: ['testBlobData']  // the test blob data which will be passed back to callback
}];

• Node

 global['__zone_symbol__FakeAsyncTestMacroTask'] = [{
     source: 'HTMLCanvasElement.toBlob',
     callbackArgs: ['testBlobData']  // the test blob data which will be passed back to callback
}];

@vikerman , please review.

And the CI will pass after #935 is merged (because rxjs 5.5.0's issue).