Add tests of implicit and explicit stdin/stdout arguments

[rrthomas]

See branch test-file-args. The test inputs will need to be discarded (as their source is unknown) and the test results regenerated with fresh inputs.

The test suite will need some additions to cope with non-file arguments.

Add an output argument to the Case class, which defaults to "output" to mean a file as at present, and can also be the string "-" to mean "use an explicit stdout argument", or None to mean "use an implicit stdout argument". Extend the input argument to also have a special interpretation for "-" and allow None.

Then in file_test, the extra values will need to be interpreted.