Run test E2E CI job on macOS as well (#424)

[SleepySquash]

Resolves #424

Synopsis

On CI E2E tests run on -d web-server currently only.

Solution

This PR adds macOS target to run E2E tests on CI.

Checklist

• Created PR:
  • In draft mode
  • Name contains issue reference
  • Has type and k:: labels applied
• Before review:
  • Documentation is updated (if required)
  • Tests are updated (if required)
  • Changes conform code style
  • CHANGELOG entry is added (if required)
  • FCM (final commit message) is posted or updated
  • Draft mode is removed
• Review is completed and changes are approved
  • FCM (final commit message) is approved
• Before merge:
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  • All temporary labels are removed

[ChabanovX]

If something wrong with Gherkin, then another approach might be to migrate some macos e2e tests to patrol. That's a good package, but available only on mobile + macos. Provides native funcitonality testing.

[SleepySquash]

@ChabanovX, thank you for a suggestion! Ideally we want to stick with BDD. We even considered https://pub.dev/packages/bdd_widget_test and https://pub.dev/packages/bdd_framework, but those had some drawbacks compared to flutter_gherkin. Honestly, I'm thinking about writing own BDD testing framework from time to time, but that would take a bit too much time.