Improve ssh-gssapi DEP8 test

The DEP8 test introduced in https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/2053146 could still show s PASS even when the login didn't work. This is because it's relying on `set -e` to work inside functions, but that's not the case.

For example, here I forced a failure by using an invalid user (I added "x" to the username):
```
## ssh'ing into localhost using gssapi-keyex auth
<email address hidden>: Permission denied (gssapi-keyex).

## checking that we got a service ticket for ssh (host/)
03/18/24 12:16:55 03/18/24 22:16:55 host/sshd-gssapi.example.fake@
 Ticket server: <email address hidden>

## Checking ssh logs to confirm gssapi-keyex auth was used
Mar 18 12:16:55 sshd-gssapi.example.fake sshd[22994]: Failed gssapi-keyex for invalid user testuser22924x from 127.0.0.1 port 39550 ssh2: <email address hidden>
## PASS test_gssapi_keyex_login

```

Furthermore, the --grep option used in journalctl is not specific enough, as can also be seen above. It's just looking for the authentication method name, not whether is succeeded or not.