Native tests for VelocityTracker::getEstimate

Earlier commit broke VelocityTracker::getEstimate, which also broke
pointer location. In additional VelocityTracker::getEstimate is an @hide
api that could be potentially accessed by app.
Add native tests for getEstimate (and specifically, for the currently
used LeastSquares lsq2 strategy) to prevent future regressions.

Bug: 114017699
Test: atest libinput_tests
Change-Id: I69d0c40283b9589b8559e698b66d758a6c838f19