tests.multifunction: Search prompt without specified start(^) and end($)

For some commands, the output may not in normal format:
nonempty_lines=['parted /dev/vdb "mklabel msdos"',
'Information: You may need to update /etc/fstab.',
'                                                [root@atest-guest ~]# ']
But the shell_prompt defined in configurations will cause unmatched
to catch the last line for prompt.

So remove the symbol "^" and "$" to match it in this condition.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>