More unit tests for default snapshot class

[xing-yang]

What happened:
Open this issue to track this comment from the code review:
#1210 (comment)
We need to add more unit tests.

What you expected to happen:

How to reproduce it:

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Environment:

• Driver version:
• Kubernetes version (use kubectl version):
• OS (e.g. from /etc/os-release):
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[gnufied]

Also can we add unit test for this scenario?

1. Create a volumegroupsnapshotclass.
2. Create one or more PVCs that use volumes.
3. Take snapshot of those PVCs via volumegroupsnapshot
4. Expect everything to work. Expect volumesnapshot objects to be ready.