confusing error output when giving invalid test file name

[StefanKarpinski]

This has tripped me up several times recently. I ran something like julia test/runtests.jl string and got a confusing parade of errors instead a clear and immediate failure saying that string is not a valid test file. This is exacerbated by the fact that if you give multiple test file then those will run and the non-existent test file is reported a single test error under that name. I would propose that if you give a non-existent test file name runtests.jl should just bail immediately and run no tests.