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 for clarity, it would actually be a lot better to have jujuc.NewCommand() be the thing that is adding jujuc.CmdSuffix.
What this code cares about is calling "action-get", it doesn't care about extensions, etc.
You can think of the name there as just a handle to what you want, and it is *far* easier to read code that looks like:
 jujuc.NewCommand(context, "action-get")
than
 jujuc.NewCommand(context, "action-get"+jujuc.CmdSuffix)