add classes to buttons in confirm-dialog

Makes testing using confirm-dialog easier

Change-Id: I0caa6eda784cb3e6df8f815e7097ce9eac9129d1
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/263941
Reviewed-by: Leandro Lovisolo <lovisolo@google.com>
diff --git a/infra-sk/modules/confirm-dialog-sk/confirm-dialog-sk.js b/infra-sk/modules/confirm-dialog-sk/confirm-dialog-sk.js
index a0d7c03..bc27eb5 100644
--- a/infra-sk/modules/confirm-dialog-sk/confirm-dialog-sk.js
+++ b/infra-sk/modules/confirm-dialog-sk/confirm-dialog-sk.js
@@ -32,8 +32,8 @@
   <h2>Confirm</h2>
   <div class=message>${ele._message}</div>
   <div class=buttons>
-  <button @click=${ele._dismiss}>Cancel</button>
-  <button @click=${ele._confirm}>OK</button>
+  <button class=dismiss @click=${ele._dismiss}>Cancel</button>
+  <button class=confirm @click=${ele._confirm}>OK</button>
   </div>
 </dialog>`;