Add new tests for FeatureOverlay

In the process of creating these tests, a bug with setting this._style
was also fixed.
Add ol.renderer.vector.getSquaredTolerance and ol.SIMPLIFY_TOLERANCE

In the process of this change, it turned out that the replay needs the
squared tolerance instead of the tolerance as constructor argument,
because ol.geom.flat.simplify.snap, which is used by the replay code,
also takes the squared tolerance as argument.