Replace assert* involving an instanceof operator with assertInstanceOf()/assertNotInstanceOf()

Problem/Motivation

As title

Proposed resolution

Recommended regex for searching: assert\w+\(.+instanceof.+\)

Example:

-    $this->assertTrue($property instanceof StringInterface, 'Got the right wrapper fo the page.front property.');
+    $this->assertInstanceOf(StringInterface::class, $property, 'Got the right wrapper fo the page.front property.');

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet