Fonts: the MTEXT strip test expects a short \U+ escape to keep its digits (#2954)

strips_formatting expected `\U+12` to strip to nothing. plain_mtext
dropped every unknown code up to the next `;` when that assertion was
written. #1036 limits this to the codes in takes_arg, and `\U` is not
one of them, so plain_mtext returns `+12`. The layout and paragraph
paths give the same text, and
unicode_escapes_are_decoded_in_every_parser already expects it.

The test has failed on main since both changes merged, and the failure
stops the gate at its test step. The test module now also checks the
short escape in both layout paths.

(cherry picked from commit 6f0f1fefa8)

crates/fonts/src/mtext.rs

@@ -904,7 +904,7 @@ mod tests {
         assert_eq!(plain_mtext("\\H2.5x;Big"), "Big");
         assert_eq!(plain_mtext("unterminated \\H2"), "unterminated ");
         assert_eq!(plain_mtext("\\U+0041B \\U+00E8"), "AB è");
-        assert_eq!(plain_mtext("\\U+12"), "", "a short escape is a code with an argument");
+        assert_eq!(plain_mtext("\\U+12"), "+12", "a short escape loses only the backslash and the code");
     }
 
     #[test]
@@ -928,6 +928,8 @@ mod tests {
         assert_eq!(plain_mtext("\\M+5BCFE x;y"), "\u{4ef6} x;y");
         // Malformed escapes lose only the backslash and the code.
         assert_eq!(plain_mtext("a\\U+12 b"), "a+12 b");
+        assert_eq!(lay("a\\U+12 b", 0.0).lines, vec!["a+12 b"]);
+        assert_eq!(lay("\\pxql;a\\U+12 b", 0.0).lines, vec!["a+12 b"]);
         assert_eq!(plain_mtext("a\\M+ b"), "a+ b");
     }