Replace assertions involving calls to array_key_exists() with assertArrayHasKey()/assertArrayNotHasKey()

Problem/Motivation

As title.

Proposed resolution

For example:

-    $this->assertFalse(array_key_exists("data-entity-uuid", $htmlRestrictions['allowed']['a']));
+    $this->assertArrayNotHasKey("data-entity-uuid", $htmlRestrictions['allowed']['a']);

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet