Rust: Add a test for the new string lexer.

tests/ctags/test_input.rs

@@ -6,6 +6,8 @@ use std::io::stdio::println;
 use test_input2::*;
 mod test_input2;
 
+fn preserve_string_delims(_bar: extern r#"C"# fn()) {}
+
 pub struct A
 {
 	foo: fn() -> int,

tests/ctags/test_input.rs.tags

@@ -31,6 +31,7 @@ main
 my_method128(&self,_:int)Foo0
 nested16()main0
 only_field8S10
+preserve_string_delims16(_bar: extern r#"C"# fn())0
 size163840
 some216(a:Animal)0
 test128(&self)Foo0