LibWeb: Fix endless spinning in apply_the_history_step()

While waiting for a task that populates a session history entry, we
can't limit the processing of the event loop to the
`NavigationAndTraversal` task source. This is because fetching uses the
`Networking` task source, which also needs to be processed.

Since making a fetch request might take some time, we want to process
everything on the event loop while waiting, to avoid blocking user
interactions.

It is still possible to use `spin_processing_tasks_with_source_until()`
on subsequent steps of `apply_the_history_step()`.

Also modifies test that was flaky.

Tests/LibWeb/Text/expected/navigation/remove-iframe-from-timeout-callback.txt

@@ -1 +1 @@
-  
+PASS: did not crash