Add functional tests for zvsh

[larsbutler]

zvsh provides a lot of convenience for running ZeroVM, and it has many options. However, it is not very well tested at present, which makes it tricky to safely make changes.

It was suggested by @pkit that we add some functional tests to zvsh: #31 (comment)

[mgeisler]

Do you mean more unit tests or more whole-system tests?

I like both kinds of tests, but for command line programs in particular, system testing makes sense to me. I suggest using the cram tool for some of this like we do for zpm. Tests like this one immediately revealed some whole-system problems that the more specific unit tests hadn't found.

[pkit]

I will add some, maybe cram ones maybe not.
Most of zvsh output is not on command line but in the files it creates, cram seems to be not quite good for that.

[pkit]

Added quite a lot of tests.