Simplify test case for issue 5023

By replacing the code quoted from the issue with a single comment that
more directly exposes the issue. This is to

1. Make the underlying issue easier to reason about
2. Make this test not behave upon the behaviour of indentation length
   when rewriting lines (my main motivation for this change, since it
   makes the followup patch simpler)

This was tested by partially reverting
368a9b7 with the patch:

```patch
diff --git i/src/string.rs w/src/string.rs
index 3b97118..561e70ac 100644
--- i/src/string.rs
+++ w/src/string.rs
@@ -278,9 +278,6 @@ fn break_string(max_width: usize, trim_end: bool, line_end: &str, input: &[&str]
         }
         cur_index
     };
-    if max_width_index_in_input == 0 {
-        return SnippetState::EndOfInput(input.concat());
-    }

     // Find the position in input for breaking the string
     if line_end.is_empty()
```

The re-running tests, or executing `rustmft` directly on this test,
triggered the panic from the original bug.

Author: matthewhughes934

tests/source/issue-5023.rs

@@ -1,22 +1,15 @@
 // rustfmt-wrap_comments: true
 
+// below we try and force a split at a byte in the middle of a multi-byte
+// character. The two parts of the first line are constructed such that:
+// 1) the entire line is longer than `$comment_width`
+// 2) the length of the second part is such that:
+//   `$comment_width - 3 + $length` is positive _and_ less than the number of
+//   bytes in the multi-byte char
+
+// xxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+// 是
+
 /// A comment to test special unicode characters on boundaries
 /// 是，是，是，是，是，是，是，是，是，是，是，是  it should break right here this goes to the next line
-fn main() {
-    if xxx {
-        let xxx = xxx
-            .into_iter()
-            .filter(|(xxx, xxx)| {
-                if let Some(x) = Some(1) {
-                    // xxxxxxxxxxxxxxxxxx, xxxxxxxxxxxx, xxxxxxxxxxxxxxxxxxxx xxx xxxxxxx, xxxxx xxx
-                    // xxxxxxxxxx. xxxxxxxxxxxxxxxx，xxxxxxxxxxxxxxxxx xxx xxxxxxx
-                    // 是sdfadsdfxxxxxxxxx，sdfaxxxxxx_xxxxx_masdfaonxxx，
-                    if false {
-                        return true;
-                    }
-                }
-                false
-            })
-            .collect();
-    }
-}
+fn main() {}

tests/target/issue-5023.rs

@@ -1,23 +1,17 @@
 // rustfmt-wrap_comments: true
 
+// below we try and force a split at a byte in the middle of a multi-byte
+// character. The two parts of the first line are constructed such that:
+// 1) the entire line is longer than `$comment_width`
+// 2) the length of the second part is such that:
+//   `$comment_width - 3 + $length` is positive _and_ less than the number of
+//   bytes in the multi-byte char
+
+// xxxxxxxxxx
+// xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+// 是
+
 /// A comment to test special unicode characters on boundaries
 /// 是，是，是，是，是，是，是，是，是，是，是，是  it should break right here
 /// this goes to the next line
-fn main() {
-    if xxx {
-        let xxx = xxx
-            .into_iter()
-            .filter(|(xxx, xxx)| {
-                if let Some(x) = Some(1) {
-                    // xxxxxxxxxxxxxxxxxx, xxxxxxxxxxxx, xxxxxxxxxxxxxxxxxxxx xxx xxxxxxx, xxxxx xxx
-                    // xxxxxxxxxx. xxxxxxxxxxxxxxxx，xxxxxxxxxxxxxxxxx xxx xxxxxxx
-                    // 是sdfadsdfxxxxxxxxx，sdfaxxxxxx_xxxxx_masdfaonxxx，
-                    if false {
-                        return true;
-                    }
-                }
-                false
-            })
-            .collect();
-    }
-}
+fn main() {}