REST API Unit Test Docs: Remove reference to the plugin

[kraftbj]

In the unit tests for the REST API infrastructure, a mention the "the plugin should be installed and activated", referring to when the infrastructure was in the feature plugin. Since that is merged to Core, the reference is outdated.

[rachelbaker]

In 38771:

REST API: Remove reference to the plugin in infrastructure activation test.

Also adds class exists checks for WP_REST_Request and WP_REST_Response.

Props kraftbj.
Fixes #38279.