Write tests

[dabreegster]

• [ ] Same line exactly
• [ ] Same line exactly, but reversed
• [ ] Partial overlap (https://github.com/actenglabs/overline/blob/master/test-data/lines_without_shared_vertices_overline_r.geojson)
• [ ] Two lines crossing at a single point
• [ ] Near parallel lines, slightly offset coordinates. (They shouldn't aggregate, unless we add an optional quantization parameter)
• [ ] Check what happens when a single input doubles back on itself. Route-snapper results often do this!