Make window-open-blank-from-different-initiator.html use slow.py instead

The test initially used infinite-slow-response.py to make a navigation that never commits. However, infinite-slow-response.py will actually commit as the HTTP response for the navigation started (it just never finished), unblocking the navigation from committing. To achieve the goal of having a navigation that practically never commits, we will use slow.py, which will not start the HTTP response until after a very long delay, making a navigation that's pending for a very long time.

Bug: 330011153 Change-Id: I9a4778c71474f1f612a00f184d6abaad5b2612a9 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5376394 Commit-Queue: Rakina Zata Amni rakina@chromium.org Commit-Queue: Arthur Sonzogni arthursonzogni@chromium.org Reviewed-by: Arthur Sonzogni arthursonzogni@chromium.org Cr-Commit-Position: refs/heads/main@{#1276860}