Refactor tests

[MikaelSlevinsky]

We should have as goal a 10-minute test suite per job on CI, per JuliaApproximation package. The tests in the ApproxFun ecosystem evolved from

• a collection of tricksy gotcha's to make function approximation fun and intuitive
• systematic test functions from https://github.com/JuliaApproximation/ApproxFunBase.jl/blob/master/src/testing.jl Probably there are ways to pare down the tests while ensuring the same level of support. Huge test suites needlessly clog up CI infrastructure and slow down development.