Make it possible to run subset of tests

[mkschleg]

Right now when running tests you have to run the entire suite, which takes a few minutes. Might be convenient for development if we adopt the use of test_args to allow for running specific tests (see JuliaLang/Pkg.jl#1226).

[mcabbott]

I stole your idea to make JuliaDiff/ChainRules.jl#511. But would be good to do here, too.

[ToucheSir]

Also for consideration is something more batteries included like https://github.com/JuliaTesting/ReTest.jl.

[mkschleg]

It might be interesting to go w/ a fuller featured tester for Flux, but I'm a bit apprehensive that it is not v1.0. Once it becomes stable moving to it might be good, but for now @mcabbott's include_test seems like a good option (and a relatively minimal change).

[mkschleg]

I took a light pass over this. I think to support this the way @mcabbott wrote it we would have to have all our import/using statements in the runtests.jl file. This would require a larger refactor of the tests than I had anticipated. There might be some other macro/expression magic to make it work, but that is currently beyond my skill.