The enabling of test_preserve_symlinks() test in TestFSFolder.c has heretofore
been keyed off of the existence of the unistd.h header file. This has caused
errors under Strawberry Perl, which provides the unistd.h header but without a
symlink() function. The solution is to disable the test based on whether the
_WIN32 symbol is defined.
We also add a test which uses windows.h and CreateSymbolicLink() rather than
unistd.h and symlink(). However it must remain disabled by default, since
CreateSymbolicLink() fails unless the user running the test has administrator