Use test.FlagSuite for storage tests

[plar]

Change Overview

This PR is the 8th in the series of PRs that will add a new way to build Kopia CLI commands.

PR Train:

1. PR-2653 Implement Kopia core flag handling data structures and flag testing suite
2. PR-2654 Implement Kopia common args and flags
3. PR-2655 Implement Kopia storage model and flags
4. PR-2656 Implement Kopia storage Filesystem flags
5. PR-2657 Add Kopia GCS storage flags
6. PR-2658 Add Kopia Azure storage flags
7. PR-2659 Add Kopia S3 and S3 compliant storage flags
8. PR-2660 Use test.FlagSuite for storage tests
9. PR-2661 Add Kopia repository create command
10. PR-2662 Add Kopia repository connect command
11. PR-2663 Add Kopia repository connect server command
12. PR-2664 Add Kopia repository set-parameters command
13. PR-2665 Add Kopia repository status command

This PR unifies testing storage flags using test.FlagSuite.

Pull request type

Please check the type of change your PR introduces:

• 🚧 Work in Progress
• 🌈 Refactoring (no functional changes, no api changes)
• 🐹 Trivial/Minor
• 🐛 Bugfix
• 🌻 Feature
• 🗺️ Documentation
• 🤖 Test

Test Plan

• 💪 Manual
• ⚡ Unit test
• 💚 E2E