In the test case `test_branch_mutually_exclusive_actions`, we weren't actually testing anything useful, because the interface has since changed to use subcommands instead of options. The test has been deleted in this commit, and `TestEnvironment::jj_cmd_cli_error` has been changed to return a `#[must_use]` `String` representing stderr. I also added `#[must_use]` to `TestEnvironment::jj_cmd_failure` while I was here.
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).
