Investigate moving E2E test outputs to Allure

Since our E2E test reporting is failing due to a deprecation in the Slack API (see [here](https://github.com/Flagsmith/flagsmith/actions/runs/14353196180/job/40236560375)), we should evaluate other options for test reporting, e.g. Allure.

We can look into [this](https://www.npmjs.com/package/testcafe-reporter-allure-plus) (slightly stale) project or look into converting the output into a format that Allure natively integrates with (e.g. XUnit).

We should not consider moving away from TestCafe as part of this work.