add check_diff to self_tests

Author: benluiwj

src/test/mod.rs

@@ -383,19 +383,21 @@ fn idempotence_tests() {
 fn self_tests() {
     init_log();
     let mut files = get_test_files(Path::new("tests"), false);
-    let bin_directories = vec![
-        "cargo-fmt",
-        "git-rustfmt",
-        "bin",
-        "format-diff",
-        "check_diff",
-    ];
+    let bin_directories = vec!["cargo-fmt", "git-rustfmt", "bin", "format-diff"];
     for dir in bin_directories {
         let mut path = PathBuf::from("src");
         path.push(dir);
         path.push("main.rs");
         files.push(path);
     }
+    // for crates that need to be included but lies outside src
+    let external_crates = vec!["check_diff"];
+    for external_crate in external_crates {
+        let mut path = PathBuf::from(external_crate);
+        path.push("src");
+        path.push("main.rs");
+        files.push(path);
+    }
     files.push(PathBuf::from("src/lib.rs"));
 
     let (reports, count, fails) = check_files(files, &Some(PathBuf::from("rustfmt.toml")));