Add a new function (is_command), to help implementing tests that
involve running a command. This works like is_string(), except the
string value we compare against is the output of the provided command,
and we also check the exit code of the command.
Convert the existing execl() call in volser/vos-t to use this new
function.
Change-Id: I4a75b1a0333e608da6a6cd69838350116a2503a9
Reviewed-on: https://gerrit.openafs.org/14040
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
