Fix CSS Loader Detection False Positives (#34584)

`test.css`, `test.scss` etc. will falsely match on a loader that searches for the string `test`


fixes #34583



## Bug

- [x] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [x] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `yarn lint`

packages/next/build/webpack-config.ts

@@ -1848,11 +1848,11 @@ export default async function getBaseWebpackConfig(
     }
 
     const fileNames = [
-      '/tmp/test.css',
-      '/tmp/test.scss',
-      '/tmp/test.sass',
-      '/tmp/test.less',
-      '/tmp/test.styl',
+      '/tmp/NEXTJS_CSS_DETECTION_FILE.css',
+      '/tmp/NEXTJS_CSS_DETECTION_FILE.scss',
+      '/tmp/NEXTJS_CSS_DETECTION_FILE.sass',
+      '/tmp/NEXTJS_CSS_DETECTION_FILE.less',
+      '/tmp/NEXTJS_CSS_DETECTION_FILE.styl',
     ]
 
     if (rule instanceof RegExp && fileNames.some((input) => rule.test(input))) {