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.

Before we can merge this PR, please make sure that all the following items have been checked off. If any of the checklist items are not applicable, please leave them but write a little note why.

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

[Pratyush]

Thanks for the cleanup and tests!