t6423: introduce Windows-specific handling for symlinking to /dev/null

t6423: introduce Windows-specific handling for symlinking to /dev/null

The device `/dev/null` does not exist on Windows, it's called `NUL`
there. Calling `ln -s /dev/null my-symlink` in a symlink-enabled MSYS2
Bash will therefore literally link to a file or directory called `null`
that is supposed to be in the current drive's top-level `dev` directory.
Which typically does not exist.

The test, however, really wants the created symbolic link to point to
the NUL device. Let's instead use the `mklink` utility on Windows to
perform that job, and keep using `ln -s /dev/null <target>` on
non-Windows platforms.

While at it, add the missing `SYMLINKS` prereq because this test _still_
would not pass on Windows before support for symbolic links is
upstreamed from Git for Windows.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>