Expressions: Improve test coverage

[streamich]

Improve test coverage in Expressions plugin src/plugins/expressions.

• Add unit tests for everything
• Add few integration tests for the whole expressions plugin
• Set-up test coverage reporting
• Look into moving interpreter functional snapshots to use the unified snapshot interface

Parent issue: #46909

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[ppisljar]

Thank you for contributing to this issue, however, we are closing this issue due to inactivity as part of a backlog grooming effort. If you believe this feature/bug should still be considered, please reopen with a comment.