KEP-1488: Update test coverage

[mattcary]

• One-line PR description: Update test plan to match format

• Issue link: GCE PD in-tree to CSI driver migration #1488

/assign @Priyankasaggu11929

[Priyankasaggu11929]

Thanks for the changes @mattcary. LGTM!

I don't have the appropriate permissions for adding lgtm/approve labels. @xing-yang, could you please take a look?

Thank you so much!

[Priyankasaggu11929]

/assign @johnbelamaric

This PR is updating the Test Plan section to make it even with required format, which was already approved previously in merged KEP PR #3351

[xing-yang]

/lgtm
/approve

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