gui_e2e: verify a dashboard deletion against the reloaded page

The row assertion ran straight after the confirm click, so it could be
satisfied by the empty document the browser shows while the reload is
still in flight: to_have_count(0) is true of nothing at all. Deletions
that never happened reported success, and clones piled up in
user_dashboards.mk across an entire green run.

Wait for that reload before asserting.

JIRA-Ref: CMK-37975
Change-Id: Iedd2e6c3cd2e5b351e6dc3459ab98daeb9788ab1