Make tests/shtest portable to Windows

[nicowilliams]

It runs, but fails early. For example, there's no /dev/urandom on Windows.

[jankatins]

for test failures, I pushed a commit to my repo where I enabled all tests again: https://ci.appveyor.com/project/JanSchulz/jq/build/1.0.41
https://ci.appveyor.com/project/JanSchulz/jq

[nicowilliams]

Yeah, and this is the log from my build prior to disabling the failing tests: https://ci.appveyor.com/project/nicowilliams/jq/build/1.0.1/job/fsxq3h06tt8oq448

We'll also need to add regression tests for jq bugs on Windows.

[nicowilliams]

The first problem in tests/shtest, of course, is that the warnings we're looking for have Windows line endings (\r\n).