prove.sh test fails with MinGW

[Eli Zaretskii]

The prove.sh test fails here, because the version of 'prove' I have
with MSYS is too old (1.04), and it doesn't support the -w switch.  I
do have 'prove' as part of the newer Perl I use with texi2any, but
it's a Windows batch file, so it will be difficult to invoke that from
a Bash shell script.

Do we really need the -w switch?  If I remove it, the test passes.