Narrow noExternal path check to /src/testing/ to avoid false positives

Use "/src/testing/" instead of "/testing/" in the noExternal importer
path check to avoid false positives when the project itself is located
under a directory path containing "/testing/".

#635 (comment)

Co-authored-by: Amp (https://ampcode.com)
Amp-Thread-ID: https://ampcode.com/threads/T-019d00c0-4bd0-70fe-b81f-82e3738959e9
Co-authored-by: Amp <amp@ampcode.com>

Author: dahlia

packages/fedify/tsdown.config.ts

@@ -47,7 +47,7 @@ export default [
     noExternal: (id: string, importer: string | undefined) => {
       if (id !== "@fedify/fixture") return false;
       const normalized = importer?.replaceAll(sep, "/");
-      return normalized?.includes("/testing/") ?? false;
+      return normalized?.includes("/src/testing/") ?? false;
     },
     inputOptions: {
       onwarn(warning, defaultHandler) {