test: restore symlink tests on cygwin

test/test_fcntl.rs

@@ -243,7 +243,6 @@ fn test_renameat2_noreplace() {
 
 #[test]
 #[cfg(not(target_os = "redox"))]
-#[cfg_attr(target_os = "cygwin", ignore)]
 fn test_readlink() {
     let tempdir = tempfile::tempdir().unwrap();
     let src = tempdir.path().join("a");

test/test_unistd.rs

@@ -870,7 +870,6 @@ fn test_canceling_alarm() {
 
 #[test]
 #[cfg(not(any(target_os = "redox", target_os = "haiku")))]
-#[cfg_attr(target_os = "cygwin", ignore)]
 fn test_symlinkat() {
     use nix::fcntl::AT_FDCWD;