Refactor E2E test framework

[blackpiglet]

Describe the problem/challenge you have

The current E2E framework is specific for the backup and restore workflow, but it's used for all E2E test cases now.

Describe the solution you'd like

• Refactor the framework to BRcase to make clear its scope.
• Add new framework specific for the migration case.

Anything else you would like to add:

Environment:

• Velero version (use velero version):
• Kubernetes version (use kubectl version):
• Kubernetes installer & version:
• Cloud provider or hardware configuration:
• OS (e.g. from /etc/os-release):

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