all other databases use unicode by default already, so I thought it would unify their set ups in dbfixtures. Especially that everyone seems to be using utf8 by default anyway.

Though you might be right - what's default is default. 
I'd still either move it to config, and set explicitly to latin1 and latin1_swedish_ci, or use these defaults here, to know exacly what the collation is explicitly in tests, and not depend on my.cnf setting on this or that machine?