Add fmt::Debug string escape tests

Author: semmaz

src/test/run-pass/ifmt.rs

@@ -74,6 +74,10 @@ pub fn main() {
     t!(format!("{:?}", 10_usize), "10");
     t!(format!("{:?}", "true"), "\"true\"");
     t!(format!("{:?}", "foo\nbar"), "\"foo\\nbar\"");
+    t!(format!("{:?}", "foo\n\"bar\"\r\n\'baz\'\t\\qux\\"),
+       r#""foo\n\"bar\"\r\n\'baz\'\t\\qux\\""#);
+    t!(format!("{:?}", "foo\0bar\x01baz\u{3b1}q\u{75}x"),
+       r#""foo\u{0}bar\u{1}baz\u{3b1}qux""#);
     t!(format!("{:o}", 10_usize), "12");
     t!(format!("{:x}", 10_usize), "a");
     t!(format!("{:X}", 10_usize), "A");