[Feature Request][Prism]: Add test for StateKey_MultimapSideInput in State in worker.go

[mls3odp]

What would you like to happen?

Add missing unit test for StateKey_MultimapSideInput case in prism worker.

Issue Priority

Priority: 3 (nice-to-have improvement)

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[mls3odp]

.take-issue

[lostluck]

Note: If you have Fixes # on it's own line in the PR, it will automatically close the issue on merge. "Addresses" might work too, but I think it needs to be clearly on it's own line though, rather than in the middle of other sentences.