Remove legacy hook from Testing module.

[kiamlaluno]

simpletest_registry_files_alter() is a hook_registry_files_alter() implementation Backdrop core does not invoke anymore, since the class registry and its hooks has been replaced by hook_autoload_info() and hook_autoload_info_alter().

simpletest_registry_files_alter() is supposed to add test files for disabled modules. If that is necessary, a different hook must be used; otherwise, that function can simply be removed.

[kiamlaluno]

The only failure is caused by the layout_load_multiple_by_path() function which is not found.

[herbdool]

@kiamlaluno while you're at it, can you also update the comment for simpletest_test_get_all()? It also mentions that same missing hook.

[kiamlaluno]

I also updated the PR as per the recent commits.

[quicksketch]

Excellent, thank you @kiamlaluno and @herbdool! Merged backdrop/backdrop#4624 into 1.x and 1.27.x.