Add alternate-screen scrolling test case

The screen contents must be correct, but also
the stable_index must be maintained, even when in
alternate-screen mode, where scrollback lines are
discarded.

term/src/test/mod.rs

@@ -1181,6 +1181,18 @@ fn test_1573() {
     assert_eq!(graphemes, vec![sequence]);
 }
 
+#[test]
+fn test_6166() {
+    let smcup = "\u{1b}[?1049h\u{1b}[22;0;0t";
+
+    let mut term = TestTerm::new(5, 5, 0);
+    term.print(smcup);
+    term.print("1\r\n2\r\n3\r\n4\r\n5\r\n6\r\n7\r\n8");
+    assert_eq!(term.screen().visible_row_to_stable_row(0), 3);
+
+    assert_all_contents(&term, file!(), line!(), &["4", "5", "6", "7", "8"]);
+}
+
 #[test]
 fn test_hyperlinks() {
     let mut term = TestTerm::new(3, 5, 0);