Split out test code into its own package

[ianmackenzie]

Would be useful to let other packages write tests using the elm-geometry fuzzers and expectation functions. Will probably have to do some experimenting with Git submodules etc. to find the right approach - ideally elm-geometry contributors can still just check out elm-geometry itself and get a fully working, testable version of the package, without having to do anything like also have to check out a parallel copy of an elm-geometry-test package.