Expose FixtureFactory in MagentoContext

I discovered an issue using fixtures when extending from `MagentoContext`. I found I couldn't access the `FixtureFactory` unless I extended from `RawMagentoContext` (which has a `getFixtureFactory()` method) - but that caused problems because it contains all the steps defined in `MinkContext`, which caused duplication and threw ambiguous exceptions.

My solution to this was to implement `getFixtureFactory()` in `MagentoContext`. It seems like this method was omitted originally, as all the other private properties (`$app`, `$configManager`, `$cacheManager`, `$sessionService`) have getters and setters, but `$factory` only has a setter, and then `getFixture()`. 

I'm not sure whether this omission was intentional or accidental, I think the original intention was to use `getFixture()` to return these objects, but a change in the `FixtureFactory` implementation means this no longer works.