rust-regex: add test for verbatim regex syntax

This makes sure it's not modified.

Differential Revision: https://phab.mercurial-scm.org/D8508

rust/hg-core/src/filepatterns.rs

@@ -625,6 +625,15 b' mod tests {'
             .unwrap(),
             Some(br"(?:.*/)?rust/target(?:/|$)".to_vec()),
         );
+        assert_eq!(
+            build_single_regex(&IgnorePattern::new(
+                PatternSyntax::Regexp,
+                br"rust/target/\d+",
+                Path::new("")
+            ))
+            .unwrap(),
+            br"rust/target/\d+".to_vec(),
+        );
     }
 
     #[test]