Add test for an inner ring touching the outer at two points

[brawer]

What

Adds inner_touching_outer_at_two_points_splits_it_into_two_pieces to PolygonAssembler's tests: an inner ring that touches the outer ring at exactly two points (not along an overlapping edge, not crossing), so removing it splits the outer into two disjoint polygons rather than leaving one polygon with a hole.

Why

While looking into #635, we checked whether the (2012, unlicensed, since-superseded) nimix/osm_conv_tests test suite had anything our vendored osm-testdata grid corpus doesn't. Everything distinctive in it turned out to already be covered — except this one case. grid has fixtures for single-point inner/outer touches (7/754-7/758), touching along a shared line (7/756, 7/757), and a single self-touching way resolving into two components (7/765, 7/766) — but not two separate simple rings where the inner touches the outer at exactly two points.

Also proposed upstream as osmcode/osm-testdata#11, so grid itself gains the coverage too — this test stands on its own regardless of whether that's accepted.

Testing

• New test passes; verified it exercises the intended split-into-two-pieces path (Geometry::MultiPolygon with 2 elements).
• All 86 geometry:: tests pass, clippy and fmt clean.

[brawer]

LGTM