wasmtest: fix resource cleanup and add logging

The chromedp context was not cancelled, so resources may have been leaking.
Additionally this waits for the browser to start before the timer starts, and extends the timeout to 20 seconds.
Logging from chromedp has also been enabled, which may help identify possible issues?

tests/wasm/chan_test.go

@@ -15,8 +15,7 @@ func TestChan(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	ctx, cancel := chromectx()
-	defer cancel()
+	ctx := chromectx(t)
 
 	err = chromedp.Run(ctx,
 		chromedp.Navigate(server.URL+"/run?file=chan.wasm"),