Draft: Expected fail: Notice on duplicate alter type

Closes #3533944

added 5 commits

• 9b03a82b - Test non-implemented and repeated alter types.
• 86e1ac23 - Test duplicate alter hook listeners.
• 569ef79b - Add FormAlterTest kernel test.
• 49efb5dc - Call array_unique() in alter(), to avoid notice for repeated hooks.
• 548a85cc - Trigger a notice on non-unique alter types.

Compare with previous version

added 8 commits

• 3fec1ac9 - Test non-implemented and repeated alter types.
• 02836d0f - Test duplicate alter hook listeners.
• 7f11bfc9 - Add FormAlterTest kernel test.
• 6325a206 - Test effect of non-unique alter types on ordering rules.
• 3531692c - Remove leftover part of a doc comment on assertAlterCallOrder().
• c2dc253e - Call array_unique() in getCombinedListeners(), to avoid notice for repeated hooks.
• 031f26a3 - Call array_unique() in alter() instead of in getCombinedListeners().
• 10d261dd - Trigger a notice on non-unique alter types.

Compare with previous version