[perl #21442] fix fs.t failures on Win32 FAT drive

See ticket for details. This commit does not fix all issues in #21442.
[perl #21442] only check link() error codes where link() works

Skip the tests on non-NTFS.
[perl #21442] skip link() tests where the filesystem doesn't support them
[perl #21442] on Win32, skip tests that won't work on FAT when on FAT

FAT doesn't support link() and its access time resolution is only to
the day.