We didn't have any testing of exit codes on failure, other than
checking that they were not 0. This patch changes that so we always
check. Since we have the special exit code 2 (set by `clap`) for
incorrect command line, I've replaced some testing of error messages
by testing of just the exit code.
As part of this, I also fixed `jj branch --allow-backwards` to
actually require `-r` (it didn't before because having a default value
means the argument is considered always provided).
It's annoying especially for tests to not be able to append to a
config file without knowing the contents (as you have to do with
TOML). Let's read all files in a directory if `$JJ_CONFIG` points to a
directory. Mercurial does that for its `$HGRCPATH` variable.
Sometimes it's useful to have an environment variable set for all
commands in a test. This patch lets you do that by adding environment
variables to the `TestEnvironment` itself. These will then be set on
all subsequent commands.
