fix(wpt): download files as buffer instead of text

There are binary files in the WPT repository and downloading them as
text corrupts them.

Refs: nodejs/node#37294

lib/github/tree.js

@@ -58,10 +58,10 @@ class GitHubTree {
     return `${this.repoUrl}/tree/${commit.slice(0, 10)}/${this.path}`;
   }
 
-  async text(assetPath) {
+  async buffer(assetPath) {
     await this.getLastCommit();
     const url = this.getAssetUrl(assetPath);
-    return this.request.text(url);
+    return this.request.buffer(url);
   }
 
   /**

lib/request.js

@@ -37,6 +37,10 @@ class Request {
     return wrappedFetch(url, options);
   }
 
+  async buffer(url, options = {}) {
+    return this.fetch(url, options).then(res => res.buffer());
+  }
+
   async text(url, options = {}) {
     return this.fetch(url, options).then(res => res.text());
   }