Re: [PATCH v2 1/6] t4061: use POSIX compliance regex(7)

> BRE interprets `+` literally, and
> `\+` is undefined for POSIX BRE, from:
> https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_03_02
> 
> > The interpretation of an ordinary character preceded
> > by an unescaped <backslash> ( '\\' ) is undefined, except for:
> > - The characters ')', '(', '{', and '}'
> > - The digits 1 to 9 inclusive
> > - A character inside a bracket expression
> 
> This test is failing with busybox sed, the default sed of Alpine Linux
> 
> Fix it by using literal `+` instead.
> 
> Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>