Alter hook implementations in Kernel test classes are not executed

Problem/Motivation

#3502432: Make hook testing with kernel tests very simple made it possible to implement hook methods inside Kernel test classes, so that test hooks did not need to be implemented in separate modules. This generally works for most cases.

However, when alter hooks are implemented, and there multiple related types of alter hooks invoked (such as form_alter, form_FORM_ID_alter, etc.), and there are methods implementing multiple of these types, the the kernel test class hook implementations are not executed.

This occurs because in ModuleHandler::getCombinedListeners(), as part of the process of ordering all the related hook implementations, the implementations are filtered by installed modules. Since the "module" that is defined for Kernel test hook implementations is core, the kernel hook implementations are filtered out.

Steps to reproduce

See test class in MR 15493 for an example of a kernel test class that has implements multiple "types" of the same basic hook.

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet