When testing whether symlinks are correctly checked out, examine the
`core.symlinks` configuration option to determine if symlinks are
supported in a repository, don't simply assume that Windows means that
symbolic links are not supported.

Further, when testing the expected default behavior of `core.symlinks`,
test the filesystem's support to determine if symlinks are supported.

Finally, ensure that `core.symlinks=true` fails on a system where
symlinks are actually not supported.  This aligns with the behavior of
Git for Windows.