Fix flakey next-dev integration tests (#3020)
Fixes WEB-259
This should address the known cases of flakiness in the next-dev
integration test suite. Most notably, it wraps the call to
`browser.new_page`, which not only opens a new tab, it also navigates to
the destination and waits for it to load. It also:
* Migrates test code to `anyhow::Result`, and adds `context` to the
`new_page` call should it fail again.
* Mitigates flakiness resulting from browser-side Jest timeouts by
moving the circular correctness test to a new directory called
`__flakey__`. These tests aren't run at all -- unfortunatley it's not
possible to customize Jest's timeout with this particular version of
`jest-circus-browser`. WEB-319 tracks updating this dependency, which
hasn't been maintained in a while, unlike `jest-circus` itself.
Test Plan:
* [x] Run the Rust test suite in CI until confident there are no
intermittent failures.
