BulletIntegration: use CORRADE_VERIFY to compare non-printable types.

It printed some weird pointer value, which was rather confusing.
BulletIntegration: conversion of btMatrix3x3 to Magnum types.
BulletIntegration: no need to link this test to the library.
BulletIntegration: more general MotionState implementation.

Now it sadly cannot check whether all objects have common parent, but
the check wasn't definite anyway.