Follow-up to hotwired#1454

This commit continues to migrate from `assert` usage toward Playwright's
`expect`. This commit focuses on the
`src/tests/functional/frame_tests.js` file.

Utilize Playwright's built-in [assertions][] so that functional test
assertions are consistent. The `chai`-based assertions provide similar
coverage, but not much is gained for the suite by having two competing
styles of assertion.

The majority of this diff replaces `assert.equal` with
`expect(...).toEqual()`. In some cases, text comparison is replaced with
Playwright's [toHaveText][], since that assertion bakes in
synchronization and waiting support, and has the ability to compare
contents in a way that ignores spacing differences.

[assertions]: https://playwright.dev/docs/test-assertions
[toHaveText]: https://playwright.dev/docs/api/class-locatorassertions#locator-assertions-to-have-text