iOS: Add stylus pointer tests (#191564)

Adds tests for the stylus and touch-geometry fields that currently lack
testing on iOS.

`dispatchTouches:` normalises iOS-specific UITouch objects into generic
engine `PointerData` structs. These include `force`, `majorRadius`,
`altitudeAngle` and `azimuthAngleInView:`, but those have no test
coverage. Currently, the only testing we have of `PointerData` checks a
timestamp.

`tilt` and `orientation` need particular care as they're not just
straight copies of the data. iOS measures from the surface plane such
that an angle of 0 is parallel to the surface and pi/2 is vertical.
`PointerData` measures from the surface normal such that 0 is vertical
and pi/2 is parallel to the plane.

When we migrate to the embedder API, we'll need to expose these details
via `FlutterPointerEvent`. It currently has `pressure` but none of
`tilt`, `orientation` or the radius fields, so all of this has to be
added to the embedder API before iOS can adopt it.

Issue: https://github.com/flutter/flutter/issues/112232

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