test(utils, mdx-loader, core): improve coverage (#6303)

* test(utils, mdx-loader, core): improve coverage

* windows...

* fix

packages/docusaurus-utils/src/index.ts

@@ -35,7 +35,7 @@ export * from './globUtils';
 export * from './webpackUtils';
 export * from './dataFileUtils';
 
-const fileHash = new Map();
+const fileHash = new Map<string, string>();
 export async function generate(
   generatedFilesDir: string,
   file: string,
@@ -141,7 +141,10 @@ export function addLeadingSlash(str: string): string {
 }
 
 export function addTrailingPathSeparator(str: string): string {
-  return str.endsWith(path.sep) ? str : `${str}${path.sep}`;
+  return str.endsWith(path.sep)
+    ? str
+    : // If this is Windows, we need to change the forward slash to backward
+      `${str.replace(/\/$/, '')}${path.sep}`;
 }
 
 // TODO deduplicate: also present in @docusaurus/utils-common
@@ -264,20 +267,6 @@ export function mergeTranslations(
   return contents.reduce((acc, content) => ({...acc, ...content}), {});
 }
 
-export function getSwizzledComponent(
-  componentPath: string,
-): string | undefined {
-  const swizzledComponentPath = path.resolve(
-    process.cwd(),
-    'src',
-    componentPath,
-  );
-
-  return fs.existsSync(swizzledComponentPath)
-    ? swizzledComponentPath
-    : undefined;
-}
-
 // Useful to update all the messages of a translation file
 // Used in tests to simulate translations
 export function updateTranslationFileMessages(