[Lens] Fix formula functional test

[dej611]

Summary

Fixes #102183

Removed previous fix as it was working properly and went for the usual fix for text input: delete char by char the formula when updating it.

Flaky test runner: 100 runs for the whole lens suite => https://kibana-ci.elastic.co/job/kibana+flaky-test-suite-runner/1664/

Checklist

Delete any items that are not applicable to this PR.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• Any UI touched in this PR is usable by keyboard only (learn more about keyboard accessibility)
• Any UI touched in this PR does not create any new axe failures (run axe in browser: FF, Chrome)
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser)
• This was checked for cross-browser compatibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[wylieconlon]

I haven't run this change, but we've done the same fix in a few other places. LGTM from code only.

[kibanamachine]

💚 Build Succeeded

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✅ unchanged

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[kibanamachine]

💚 Backport successful

Status	Branch	Result
✅	7.x

This backport PR will be merged automatically after passing CI.

[kibanamachine]

Looks like this PR has a backport PR but it still hasn't been merged. Please merge it ASAP to keep the branches relatively in sync.