Upgrade to mockito v5

[evanweible-wf]

Motivation

There's a new major of mockito available and we'll need to upgrade to it in order to be able to upgrade to newer analyzer versions in the future.

Changes

• Upgrade mockito to ^5.0.0
• Enable the mockito builder, which is the easiest option for generating null-safe-compatible mocks
  • Note that mocks were moved to a centralized test/mockito.dart file so that this builder could be narrowly scoped to minimize its impact on build time
• Remove unused mocks

[aviary-wf]

Security Insights

No security relevant content was detected by automated scans.

Action Items

• Review PR for security impact; comment "security review required" if needed or unsure
• Verify aviary.yaml coverage of security relevant code

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Questions or Comments? Reach out on Slack: #support-infosec.

[evanweible-wf]

Going to wait on this until we can safely raise the analyzer lower bound to 1.0.0