Expand the entity reference field formatter test coverage

Problem/Motivation

Currently, the entity reference field has almost no test coverage for its formatters. Minimal coverage will be added in #2099131: Use #pre_render pattern for entity render caching. The remaining formatters require coverage.

Proposed resolution

Add test coverage for untested formatters.

See \Drupal\entity_reference\Tests\EntityReferenceFormatterTest:testIdFormatter() for an example. We can probably move much of the setup code in the two existing test methods into the setup method to avoid duplication.

User interface changes

None.

API changes

None.

Original report by @jessebeach