Don't walk the tests/ directories in checks that always skip it

`WalkBuilder` handles top-level paths differently than `fn walk` used
to: it doesn't run the `skip` function to determine if it should be
skipped, instead assuming the top-level function is always included.

This is a reasonable assumption; adapt our code so it doesn't make
pointless calls to `walk`.

Author: jyn514

src/tools/tidy/src/edition.rs

@@ -9,27 +9,20 @@ fn is_edition_2021(mut line: &str) -> bool {
 }
 
 pub fn check(path: &Path, bad: &mut bool) {
-    walk(
-        path,
-        |path| {
-            filter_dirs(path)
-                || (path.ends_with("tests") && path.join("COMPILER_TESTS.md").exists())
-        },
-        &mut |entry, contents| {
-            let file = entry.path();
-            let filename = file.file_name().unwrap();
-            if filename != "Cargo.toml" {
-                return;
-            }
+    walk(path, |path| filter_dirs(path), &mut |entry, contents| {
+        let file = entry.path();
+        let filename = file.file_name().unwrap();
+        if filename != "Cargo.toml" {
+            return;
+        }
 
-            let is_2021 = contents.lines().any(is_edition_2021);
-            if !is_2021 {
-                tidy_error!(
-                    bad,
-                    "{} doesn't have `edition = \"2021\"` on a separate line",
-                    file.display()
-                );
-            }
-        },
-    );
+        let is_2021 = contents.lines().any(is_edition_2021);
+        if !is_2021 {
+            tidy_error!(
+                bad,
+                "{} doesn't have `edition = \"2021\"` on a separate line",
+                file.display()
+            );
+        }
+    });
 }

src/tools/tidy/src/main.rs

@@ -91,7 +91,6 @@ fn main() {
 
         // Checks that need to be done for both the compiler and std libraries.
         check!(unit_tests, &src_path);
-        check!(unit_tests, &tests_path);
         check!(unit_tests, &compiler_path);
         check!(unit_tests, &library_path);
 
@@ -107,7 +106,6 @@ fn main() {
         check!(edition, &src_path);
         check!(edition, &compiler_path);
         check!(edition, &library_path);
-        check!(edition, &tests_path);
 
         check!(alphabetical, &src_path);
         check!(alphabetical, &tests_path);

src/tools/tidy/src/unit_tests.rs

@@ -24,7 +24,6 @@ pub fn check(root_path: &Path, bad: &mut bool) {
         let file_name = path.file_name().unwrap_or_default();
         if path.is_dir() {
             filter_dirs(path)
-                || path.ends_with("tests")
                 || path.ends_with("src/doc")
                 || (file_name == "tests" || file_name == "benches") && !is_core(path)
         } else {