Adding tests for ls. Corrected issue with ls on the present directory throwing panic!()

tests/ls.rs

@@ -0,0 +1,16 @@
+#[macro_use]
+mod common;
+
+use common::util::*;
+
+static UTIL_NAME: &'static str = "ls";
+
+#[test]
+fn test_ls_ls() {
+    let (at, mut ucmd) = testing(UTIL_NAME);
+    
+    let result = ucmd.run();
+    
+    let exit_success = result.success;
+    assert_eq!(exit_success, true);
+}