mingw: skip a test in t9130 that cannot pass on Windows

mingw: skip a test in t9130 that cannot pass on Windows

On Windows, Git itself has no clue about POSIX paths, but its shell
scripts do. In this instance, we get mixed paths as a result, and when
comparing the path of the author file, we get a mismatch that is
entirely due to the POSIX path vs Windows path clash.

Let's just skip this test so that t9130-git-svn-authors-file.sh passes
in Git for Windows' SDK.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>