Support running test cases parallelly

[blackpiglet]

Describe the problem/challenge you have

The Velero E2E test duration is more than 4 hours. It's better to save some time to make the CI pipeline run more efficiently.

https://onsi.github.io/ginkgo/index#spec-parallelization
Ginkgo v2 supports running test cases parallelly. We can leverage that feature to reduce this time consumption.

Describe the solution you'd like

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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