Unit tests: volume group snapshot builder

[leonardoce]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug

/kind cleanup

/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:

Enhance the readability of volume group snapshot unit tests by removing the newGroupSnapshotContent and the newGroupSnapshot functions which had way too many arguments, as referred in a previous code review.

To make the review easier, this patch addresses only the group snapshot functions, a next one will clean up newSnapshot and newContent too, fully addressing #1232.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

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