Add Grain LFSR test cases

[Insun35]

Description

• Refactoring: replace unnecessarily duplicated code in the init() to calling update().
• Test: add more test cases for get_field_elements_mod_p in the Grain LFSR consistency test.

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• Targeted PR against correct branch (main)
• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

[Pratyush]

Thanks for the cleanup and tests!