Add test for elb help error

This is a CLI specific test for #622.

The actual fix for this is in https://github.com/boto/bcdoc/pull/27, but this ensures that elb help doesn't contain invalid/empty links.

Note that this commit also redirects stderr to a pipe, so we catch any groff warnings printed to stderr.  I noticed this when groff reads from stdin that it will emit a warning complaining about the long link name.