Dual class testing meta issue

This is a testing issue to enumerate the possible different dual class combinations and keep track of related issues.
Some pre-generated dual class ready characters are included, which can be compared with the original game. One reason for this is so I can be sure I didn't break anything with the changes I made in #1067 , but also I can document and fix any other errors I find.

Known issues
- [ ] #328 Gaining multiple levels at once with the last one activating a dual grants HP for all of them.
- [ ] #342 Imoen can cast too many spells after dual-class
- [x] #819 Berserker to mage dual class goes wrong

Pure class DC combinations

- [ ] Fighter/Cleric
- [ ] Fighter/Druid
- [x] Fighter/Mage
- [x] Fighter/Thief
- [ ] Cleric/Fighter
- [ ] Cleric/Mage
- [ ] Cleric/Ranger
- [ ] Cleric/Thief
- [ ] Mage/Cleric
- [ ] Mage/Fighter
- [ ] Mage/Thief
- [ ] Thief/Cleric
- [ ] Thief/Fighter
- [x] Thief/Mage
- [ ] Ranger/Cleric
- [ ] Druid/Fighter

Kitted DC combinations tested (not all possible ones tested, just a sample from each base class)

- [x] Druid Avenger -> Fighter
- [ ] Ranger Beastmaster -> Cleric
- [ ] Fighter Kensai
- [ ] Fighter Berserker
- [ ] Cleric of Lathander -> Fighter, Ranger, Mage : Boon of Lathander and Hold Undead abilities given too much?
- [ ] Swashbuckler -> Figher, Cleric, Mage: Original and Fighter profs don't seem to stack
- [ ] Wild mage

Testing files used

[Newgame override script to skip Irenicus cutscene](https://github.com/gemrb/gemrb/files/5551620/NEWGAME.zip)
[Dual class ready characters with 17 required stats](https://github.com/gemrb/gemrb/files/5551532/dual.class.ready.characters.zip)