[gdb/testsuite] Fix gdb.server/ext-run.exp in container

When running the gdb testsuite inside a container, I run into:
...
(gdb) gdb_expect_list pattern: /1 +root +[/a-z]*(init|systemd)/
FAIL: gdb.server/ext-run.exp: get process list (pattern 2)
...
because there's no process with pid 1 and cmd init or systemd.

In the host system (where the test passes), I have:
...
$ ps -f 1
UID        PID  PPID  C STIME TTY      STAT   TIME CMD
root         1     0  0 Sep25 ?        Ss     0:03 /usr/lib/systemd/systemd ...
...
but in the container instead:
...
UID        PID  PPID  C STIME TTY      STAT   TIME CMD
root         1     0  0 11:45 pts/0    Ss     0:00 /bin/bash
...

Fix this by also accepting bash as a valid cmd.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>