tests: added test for demoing capture names

See also: #399

Author: fulara

tests/replace.rs

@@ -39,3 +39,6 @@ replace!(match_at_start_replace_with_empty, replace_all, r"foo", "foobar", t!(""
 
 // See https://github.com/rust-lang/regex/issues/393
 replace!(single_empty_match, replace, r"^", "bar", t!("foo"), "foobar");
+
+// See https://github.com/rust-lang/regex/issues/399
+replace!(capture_longest_possible_name, replace_all, r"(.)", "b", t!("${1}a $1a"), "ba ");