Add unit tests for expressions

[AdRiley]

Pull Request Description

Adds java tests to the expression part of the code. Relies heavily on mocking as a way to understand how the API works.

Full end to end tests already exist at enso/test/Table_Tests/src/Common_Table_Operations/Expression_Spec.enso

Important Notes

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• The documentation has been updated, if necessary.
• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
  Scala,
  Java,
  TypeScript,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.
• Unit tests have been written where possible.
• If meaningful changes were made to logic or tests affecting Enso Cloud integration in the libraries,
  or the Snowflake database integration, a run of the Extra Tests has been scheduled.
  • If applicable, it is suggested to paste a link to a successful run of the Extra Tests.

[hubertp]

nit re sbt build