Add test helpers for some of the components

[AndreJoaquim]

Right now, it's a bit difficult to use some components in acceptance tests due to their complexity.
For example, the uni-mobile-number requires the following code:

await fillIn('.uni-mobile-number .uni-select', 'PT');
await fillIn('.uni-mobile-number input[type="number"]', '960000000');
await triggerKeyEvent('.uni-mobile-number input[type="number"]', 'keyup', KeyCodesType.ENTER);

This proposal aims at providing a new helper that looks like the following:

await fillInMobileNumber('PT', '960000000');

This will really ease the development and decrease the time spent in figuring out how to trigger the complete behaviour of a component

[migueldemoura]

Same problem for the uni-datepicker-input.