atf-check: Add shortcut for not-exit:0

[des]

Summary

Allow the command to be preceded by an exclamation point, which changes
the default expected result to a clean exit with a non-zero exit code.

MFC after: 1 week

[ngie]

This is a really nice, simple feature request -- I love it!

The concern I have about this change is that ! is a shell builtin operator and this could cause confusion/result in bugs if someone tries to fun the command on the command line in certain shells where ! has special meanings in different contexts (we can't always assume atf-sh is running /bin/sh, unfortunately..).

How about atf_check -s '!0' false instead? If things were done that way the negation operator could be extended to -e and -o without any sort of ambiguity/confusion.

[des]

In D58099#1333191, @ngie wrote:

The concern I have about this change is that ! is a shell builtin operator and this could cause confusion/result in bugs if someone tries to fun the command on the command line in certain shells where ! has special meanings in different contexts (we can't always assume atf-sh is running /bin/sh, unfortunately..).

Can you provide an example?

How about atf_check -s '!0' false instead? If things were done that way the negation operator could be extended to -e and -o without any sort of ambiguity/confusion.

No, that completely negates the point of the patch.