test-bzr: do not use unportable sed '\+'

Using sed -e '/[0-9]\+//' to find "one or more digits" is not
portable.

Use the Basic Regular Expression '/[0-9][0-9]*//' instead.

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>