More verbose `atf-check`.

[Duncaen]

I've been converting my test scripts to use atf_check -- command instead of running it and then checking the exit status atf_check_equal $? 0, making those tests shorter and easier to extend with additional checks that atf-check provides.

The only downside as of now is that it makes it harder to debug test scripts because atf-check hides stdout and stderr for successful executions.

It would be nice if there was some options to make atf-check more verbose.

1. A command line argument to atf-check like -v to always make it verbose (kyua already hides the output for successful tests).
2. Maybe an environment variable like ATF_DEBUG so that kyua debug could set it to allow easier debugging without having to change the test script.

I would be open to contributing those changes if they are likely to be accepted.