Suggest pattern in meaningful test diagnostic

As the reason for tidy failing the test name may seem slightly opaque,
provide a suggestion on how to rename the file.

There are some tests which merely would require reordering of the name
according to the rule. I could modify the diagnostic further to identify
those, but doing such would make it prone to bad suggestions. I have opted
to trust contributors to recognize the diagnostic is robotic, as the
pattern we are linting on is easier to match if we do not speculate on what
parts of the name are meaningful: sometimes a word is a reason, but
sometimes it is a mere "tag", such as with a pair like:

    issue-314159265-blue.rs
    issue-314159265-red.rs

Starting them with `red-` and `blue-` means they do not sort together,
despite being related, and the color names are still not very descriptive.
Recognizing a good name is an open-ended task, though this pair might be:

    colored-circle-gen-blue.rs
    colored-circle-gen-red.rs

Deciding exactly how to solve this is not the business of tidy,
only recognizing a what.

src/tools/tidy/src/ui_tests.rs

@@ -148,17 +148,17 @@ pub fn check(path: &Path, bad: &mut bool) {
             if ext == "rs" {
                 lazy_static! {
                     static ref ISSUE_NAME_REGEX: Regex =
-                        Regex::new(r"^issues?[-_]?\d{3,}").unwrap();
+                        Regex::new(r"^issues?[-_]?(\d{3,})").unwrap();
                 }
 
-                if ISSUE_NAME_REGEX.is_match(testname) {
+                if let Some(test_name) = ISSUE_NAME_REGEX.captures(testname) {
                     // these paths are always relative to the passed `path` and always UTF8
                     let stripped_path = file_path.strip_prefix(path).unwrap().to_str().unwrap();
                     if !allowed_issue_filenames.remove(stripped_path) {
                         tidy_error!(
                             bad,
-                            "UI test `{}` should use a name that describes the test and link the issue in a comment instead.",
-                            file_path.display(),
+                            "file `{stripped_path}` must begin with a descriptive name, consider `{{reason}}-issue-{issue_n}.rs`",
+                            issue_n = &test_name[1],
                         );
                     }
                 }