fix(functional-tests): Improve specificity of settings heading locator

[vpomerleau]

Because

• The locator for the settings heading, intended to confirm that the Settings page is viewed, is not specific enough and can match headings on other pages where they include "to continue to account settings"
• This can cause false positives on functional-tests.

This pull request

• Update the locator.
• Update one test that was using an assertion for the wrong page (settings instead of index)

Issue that this pull request solves

Closes: #FXA-9991

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• [x] If applicable, I have modified or added tests which pass locally.
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• [ ] I have verified that my changes render correctly in RTL (if appropriate).

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[vpomerleau]

Agree we can iterate on this, but at least we should avoid false positives moving forwards.

[pdehaan]

"I don't have a suggestion for a better locator, the settings page doesn't have something super distinct to hook on to."

Should we add something better to hook on to in the front end? Like, if we could change the front end to pass in custom data-testid="" values via <AppLayout> or something if we're trying to assert we're on a specific page?

[Trinaa]

@pdehaan we've been trying to move away from test IDs and implementation in locators if possible, but you are right that in some cases this might be the better solution.