Fixed command not found error during tests on windows

Added missing cmdSuffix in tests. Previously it was used only in the actual code. This fixes quite a number of tests on Windows.
I think gofmt should turn this into "action-get" + jujuc.CmdSuffix - although I could be wrong. Comment applies the other lines as well.

I'm not familiar with the details at play in the windows world. Can you think of a way that would not require us to export CmdSuffix and call jujuc.NewCommand(hctx, "action-get") but still end up with the correct command? Maybe automaticall appending the suffix within the NewCommand implementation? I'm not sure though, maybe that feels a bit too much like magic. What are your thoughts?