test: improve WPT report runner

Add incremental report writing after each spec completes and on
worker errors so that reports survive if the process is killed
before the exit handler runs.

Add bytes() method to readAsFetch() to match the Response API
used by newer WPT tests.

Author: panva

test/common/wpt.js

@@ -223,6 +223,7 @@ class ResourceLoader {
     return {
       ok: true,
       arrayBuffer() { return data.buffer; },
+      bytes() { return new Uint8Array(data); },
       json() { return JSON.parse(data.toString()); },
       text() { return data.toString(); },
     };
@@ -721,6 +722,7 @@ class WPTRunner {
           // Mark the whole test as failed in wpt.fyi report.
           reportResult?.finish('ERROR');
           this.inProgress.delete(spec);
+          this.report?.write();
         });
 
         await events.once(worker, 'exit').catch(() => {});
@@ -787,6 +789,9 @@ class WPTRunner {
         }
       }
 
+      // Write the report on clean exit. The report is also written
+      // incrementally after each spec completes (see completionCallback)
+      // so that results survive if the process is killed.
       this.report?.write();
 
       const ran = queue.length;
@@ -873,6 +878,9 @@ class WPTRunner {
       reportResult?.finish();
     }
     this.inProgress.delete(spec);
+    // Write report incrementally so results survive even if the process
+    // is killed before the exit handler runs.
+    this.report?.write();
     // Always force termination of the worker. Some tests allocate resources
     // that would otherwise keep it alive.
     this.workers.get(spec).terminate();