Removed utf8 encoding (#1676)

The utf8 encoding breaks images from loading. Only noticed because I've built a small image editor and needed to load the image into canvas. Using this plugin broke some of my tests. Removing it fixes the issue and doesn't appear to have any negative effects.

More details here modernweb-dev/web#980

plugins/web-test-runner-plugin/plugin.js

@@ -37,7 +37,7 @@ To Resolve:
         return;
       }
       const reqPath = request.path;
-      const result = await server.loadUrl(reqPath, {isSSR: false, encoding: 'utf8'});
+      const result = await server.loadUrl(reqPath, {isSSR: false});
       return {body: result.contents, type: result.contentType};
     },
     transformImport({source}) {