Deduplicate code in elm tests

[jonathanmaw]

Context

All the tests in tests-integration/spark/elm-tests/tests/Test*.elm follow a very similar format with minor differences, but take a lot of lines to do so.

It would be better if we could reduce the amount of duplicated code here, so that if we need to make changes that affect every test, we don't need to change the source files for every individual test.

It would also make it easier to write new tests.

Actions

• [ ] Identify which parts are the same through all/most tests.
• [ ] Write some testing utility functions which can be imported and called instead of writing many lines of code.