Use string representations of expressions in `CoalescerTest` and `ApiUiContentEntityReferenceControllersTest`
> **Follow-up of** [!1112](https://git.drupalcode.org/project/canvas/-/merge_requests/1112) — see [note 1200328](https://git.drupalcode.org/project/canvas/-/merge_requests/1112#note_1200328).
### Overview
`CoalescerTest` and `ApiUiContentEntityReferenceControllersTest` express their expectations as `*PropExpression` object constructions. These are harder to scan than the equivalent string representations. A string representation is, post learning-curve, simpler for a human to read and preferable in test expectations.
### Proposed resolution
Convert the expectation values in both tests from `*PropExpression` object instantiations to their string representations, improving readability of the test cases.
### Remaining tasks
- Replace object-construction expectations with string representations in `CoalescerTest`.
- Do the same in `ApiUiContentEntityReferenceControllersTest`.
issue