LiveViewTest reports flash present when navigating between views in different sessions, when it is not

[sevenseacat]

Environment

• Elixir version (elixir -v): Elixir 1.15.7 (compiled with Erlang/OTP 26)
• Phoenix version (mix deps): phoenix 1.7.11
• Phoenix LiveView version (mix deps): phoenix_live_view 0.20.12
• Operating system: MacOS Sonoma 14.2.1
• Browsers you attempted to reproduce this bug on (the more the merrier): Firefox, Safari
• Does the problem persist after removing "assets/node_modules" and trying again? Yes/no: Yes

Actual behavior

See commit in reproduction app: sevenseacat/test_flash_session_bug@81e8964

(I put it in an app because I couldn't work out how to write/run tests in a single-file app)

Flash messages are not passed between LiveViews mounted in different sessions (verified by running the sample app and clicking from /one to Two) yet the test reports that the flash is rendered on the page.

Expected behavior

This test sevenseacat/test_flash_session_bug@81e8964#diff-2e5c87a4402d040320c823b9f1664bf3a42b06d0e66ba98db0a0fe1488867997R5-R15 should pass because the flash is not present on the page when viewed normally, but is present when run during the test.