Better tests and testsets

[asinghvi17]

This stops the testset spam, or at least reduces it, by using context instead. @testset let ... creates a special testset that doesn't instantiate itself, meaning that each @testset_implementations is a single testset, not four testsets wearing a hoodie.

Output now looks like this:

p50 union p51 - Intersection polygons with opposite winding orders and repeated points: Test Failed at /Users/anshul/.julia/dev/geo/GeometryOps.jl/test/methods/clipping/polygon_clipping.jl:209
  Expression: compare_GO_LG_clipping(GO_f, LG_f, p1, p2)
     Context: Testing geometry from module = GeoInterface

and

 Union                                                                                                                          |  188     4    192  1.4s
    p1 union p1 - Same polygon                                                                                                   |    4            4  0.1s
    p1 union p2 - Convex polygons that intersect (diamonds, four vertices)                                                       |    4            4  0.0s
    p3 union p4 - Convex polygons that intersect (randomly generated, many edges)                                                |    4            4  0.0s
    p5 union p6 - Convex polygons that intersect (randomly generated, many edges)                                                |    4            4  0.0s
    p7 union p8 - Concave polygons that intersect (randomly generated, many edges)                                               |    4            4  0.0s
    p5 union p7 - Convex and concave polygons intersect                                                                          |    4            4  0.0s
    p9 union p10 - Figure 10 from Greiner Hormann paper                                                                          |    4            4  0.0s

which is substantially better than what we had before.

This also adds a module that densifies geoms when converting to its own geomtype (:O) so we can test the strtree stuff.