[gdb/testsuite] Fix prompt regexp in batch-preserve-term-settings.exp

On openSUSE Leap 15.2, when running test-case
gdb.base/batch-preserve-term-settings.exp I get:
...
spawn /bin/sh^M
PS1="gdb-subshell$ "^M
sh-4.4$ PS1="gdb-subshell$ "^M
gdb-subshell$ PASS: gdb.base/batch-preserve-term-settings.exp: batch run: \
  spawn shell
...
but on Ubuntu 18.04.5, I get instead:
...
spawn /bin/sh^M
PS1="gdb-subshell$ "^M
$ gdb-subshell$ FAIL: gdb.base/batch-preserve-term-settings.exp: batch run: \
  spawn shell (timeout)
...

Fix this by making the regexp recognize the second pattern as well.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-12-16  Tom de Vries  <tdevries@suse.de>

	* gdb.base/batch-preserve-term-settings.exp: