[meta] Replace assertions involving calls to isset() with more appropriate assertions

Problem/Motivation

As title

Proposed resolution

Example:

-    $this->assertTrue(isset($ids['entity_test.entity_test.field_test_import']));
+    $this->assertNotNull($ids['entity_test.entity_test.field_test_import']);

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet