Resolve flapping announcement tests

[ConnorSheremeta]

These tests are currently flapping because there are some other announcements that are sometimes created in previous tests. We need to clear all announcements in a setup block.

[pgwillia]

Is this one or is this related?

Run options: --seed 51227

# Running:

Capybara starting Puma...
* Version 5.0.4 , codename: Spoony Bard
* Min threads: 0, max threads: 4
* Listening on http://127.0.0.1:37135
......[Screenshot]: /home/runner/work/jupiter/jupiter/tmp/screenshots/failures_test_should_be_able_to_login_as_a_regular_user.png
F

Failure:
AdminUsersShowTest#test_should_be_able_to_login_as_a_regular_user [/home/runner/work/jupiter/jupiter/test/system/admin_users_show_test.rb:119]:
expected to find visible css "h1" with text "Users" but there were no matches. Also found "Dashboard", which matched the selector but not all filters. 

rails test test/system/admin_users_show_test.rb:107

.......................S............

Finished in 116.591905s, 0.3688 runs/s, 3.9282 assertions/s.
43 runs, 458 assertions, 1 failures, 0 errors, 1 skips

[ConnorSheremeta]

Hmm doesn't look to be related at first glance. I'm curious now what the the capybara screenshot looks like now, might show what happened!