remove dependency on Test, Random

[StefanKarpinski]

There's no actual dependency of TranscodingStreams on Test or Random: it's only the testtools.jl file which uses them, which is not necessary for normal functioning, just for testing. It would be good to refactor things to eliminate this dependency and allow test code to load testtools.jl as necessary instead.

[musm]

Yeah agreed. I don't see why that file is under src I think we should just move it to test/testtools.jl

[nhz2]

This was fixed by #152