In ths security related issue the go1.21.4 stdlib changed the parsing
of volume names on Windows.
https://github.com/golang/go/issues/63713
This had the consequences of breaking the MkdirAll tests which were
looking for specific error messages which changed and using invalid
paths.
In particular under go1.21.3:
filepath.VolumeName(`\\?\C:`) == `\\?\C:`
But under go1.21.4 it is:
filepath.VolumeName(`\\?\C:`) == `\\?`
The path `\\?\C:` isn't actually a valid Windows path. I reported this
as a FYI bug upstream - I'm not expecting it to be fixed.
See: https://github.com/golang/go/issues/64101
This replaces built-in os.MkdirAll with a patched version that stops the recursion
when reaching the volume part of the path. The original version would continue recursion,
and for extended length paths end up with \\? as the top-level directory, and the error
message would then be something like:
mkdir \\?: The filename, directory name, or volume label syntax is incorrect.
