Changed booleanCrosses with two input lines to recognise if crossing point is between vertices

[smallsaucepan]

Previously, when checking the intersection of two lines booleanCrosses would only check candidate intersection points against line vertices, which lead to valid crossing points being missed and to false negatives.

This change has the function be more optimistic and only rule out candidate intersections if they coincide with the end points (or "boundary") of the lines. The crossing points can otherwise be anywhere on the lines.

Added tests to better exercise the different variations crosses needs to cater for, at least for two lines. Renamed a couple of other tests to better describe what they're testing.

Resolves #1901
Resolves #2967

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