tests ~ fix WSL testing

+ ToDO added ~ when possible, test under WSL2 and differentiate/liberalize if possible

tests/test_chgrp.rs

@@ -98,7 +98,10 @@ fn test_preserve_root_symlink() {
 #[test]
 #[cfg(target_os = "linux")]
 fn test_reference() {
-    if get_effective_gid() != 0 {
+    // skip for root or MS-WSL
+    // * MS-WSL is bugged (as of 2019-12-25), allowing non-root accounts su-level privileges for `chgrp`
+    // * for MS-WSL, succeeds and stdout == 'group of /etc retained as root'
+    if !(get_effective_gid() == 0 || is_wsl()) {
         new_ucmd!()
             .arg("-v")
             .arg("--reference=/etc/passwd")