This way `std.fs.symlinkAbsolute` becomes cross-platform and we can
legally include `SymlinkFlags` as an argument that's only used on
Windows. Also, now `std.os.symlink` generates a compile error on
Windows with a message to instead use `std.os.windows.CreateSymbolicLink`.
Finally, this PR also reshuffles the tests between `std.os.test` and
`std.fs.test`.
fc7d87fef1