Support for MultiApp testing XCUI

[saikrishna321]

With respect to PR appium/appium-xcuitest-driver#576 we can get this support on client once its merged

[TikhomirovSergey]

I suppose it is the candidate for the next build.

[mykola-mokhnach]

don't be so fast - these endpoints are anyway available via executeScript "mobile" interface and they should be tested properly first before creating high-level wrappers for them

[TikhomirovSergey]

@mykola-mokhnach of cource

[SrinivasanTarget]

#824