Fix POSIX::mblen mbstate_t initialization on threaded perls with glibc

As reported in https://bugs.launchpad.net/bugs/1818953 POSIX::mblen()
is broken on threaded perls with glibc.

  % perl -MPOSIX=mblen -e 'mblen("a", 1)'
  perl: mbrtowc.c:105: __mbrtowc: Assertion `__mbsinit (data.__statep)' failed.
  zsh: abort (core dumped)  perl -MPOSIX=mblen -e 'mblen("a", 1)'

This broke in v5.27.8-134-g6c9ff7e96e which made the function
use mbrlen(3) under the hood on threaded perls.

The problem is initialization of the shift state with

  mbrlen(NULL, 0, &ps));

The glibc documentation for mbrlen(3) at

  https://www.gnu.org/software/libc/manual/html_node/Converting-a-Character.html#Converting-a-Character

does not mention initialization by passing in a null pointer for the
string, only a pointer to a NUL wide character.

   If the next multibyte character corresponds to the NUL wide character,
   the return value is 0. If the next n bytes form a valid multibyte
   character, the number of bytes belonging to this multibyte character
   byte sequence is returned.

Use memset(3) instead for mbstate_t initialization, as suggested in

  https://www.gnu.org/software/libc/manual/html_node/Keeping-the-state.html

with the hope that this is more portable.

While at it, add a few basic test cases. These are in a new file because
they need fresh_perl_is() from test.pl while the existing ones use
Test::More (and conversion of at least posix.t looks way too involved.)

Bug-Ubuntu: https://bugs.launchpad.net/bugs/1818953