Move hooks from link_generation_test into the Kernel test

Problem/Motivation

In the past, if we wanted to test the behavior of a hook function, then we needed to implement that hook inside of a "test module." Test modules are typically located in core/modules/*/tests/modules/ directories.

#3502432: Make hook testing with kernel tests very simple provided an alternative to creating test modules for Kernel tests. Now hooks may be created as functions directly in Kernel test classes. This reduces the amount of boilerplate necessary for testing hooks, couples the hook function tightly with its related test, and gives us an opportunity to remove some test modules from Core.

The LinkGenerationTestHooks class (core/modules/system/tests/modules/link_generation_test/src/Hook/LinkGenerationTestHooks.php) has a hook function that appears to only be used by a Kernel test. Move the hook function into its related test.

See the following for helpful information on making this change:

• The change record about Kernel test hooks
• #3502432: Make hook testing with kernel tests very simple
• This commit contains an example of a moved hook function from a similar issue

Proposed resolution

• Search for the test class or classes that use the link_generation_test module using any method you prefer, for example grep or an IDE's search.
• Verify that the hook is only used by one or more Kernel tests. Comment if you believe this is not true.
• Copy the hook function from the test module into the Kernel test class(es).
• Delete the hook function. If the Hook class is empty, delete the entire class. If the test module has no other code, delete the entire module.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet