test(utils): add boundary tests for Older::check_older and After::check_after

[KrishnaShuk]

Description

Addresses the // TODO: test >= / > on line 111 of utils.rs. Adds 11 boundary-condition tests for the Satisfier impls of Older and After.

Notes to the reviewers

Uses Satisfier::<PublicKey>::check_older(...) fully-qualified syntax because the trait is generic over Pk.

Changelog notice

Checklists

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• I've signed all my commits
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New Features:

• I've added tests for the new feature
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Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR