cp: Fixed POSIXLY_CORRECT test to actually check if file exists and its contents (uutils#10247)

cp: Added check of file contents in POSIXLY_CORRECT test

cp: Fixed windows test by including path

cp: Updated test

Author: max-amb

tests/by-util/test_cp.rs

@@ -2991,11 +2991,15 @@ fn test_copy_through_dangling_symlink() {
 fn test_copy_through_dangling_symlink_posixly_correct() {
     let (at, mut ucmd) = at_and_ucmd!();
     at.touch("file");
+    at.write("file", "content");
     at.symlink_file("nonexistent", "target");
     ucmd.arg("file")
         .arg("target")
         .env("POSIXLY_CORRECT", "1")
         .succeeds();
+    assert!(at.file_exists("nonexistent"));
+    let contents = at.read("nonexistent");
+    assert_eq!(contents, "content");
 }
 
 #[test]