YANG parser BUGFIX reading escaped characters in quoted strings

The \t and \n sequence is actually stored as a single character so it
modifies the resulting and to store it, a dynamically allocated buffer
is needed.
diff --git a/tests/src/test_parser_yang.c b/tests/src/test_parser_yang.c
index d0595ff..cb9e058 100644
--- a/tests/src/test_parser_yang.c
+++ b/tests/src/test_parser_yang.c
@@ -322,9 +322,10 @@
 
     str = "\"hello\\n\\t\\\"\\\\\";";
     assert_int_equal(LY_SUCCESS, get_argument(&ctx, &str, Y_STR_ARG, NULL, &word, &buf, &len));
-    assert_null(buf);
+    assert_non_null(buf);
     assert_int_equal(9, len);
-    assert_string_equal("hello\\n\\t\\\"\\\\\";", word);
+    assert_string_equal("hello\n\t\"\\", word);
+    free(buf);
 
     ctx.indent = 14;
     str = "\"hello \t\n\t\t world!\"";