Make contrib regression tests safe for Danish locale.

In btree_gin and citext, avoid some not-particularly-interesting
dependencies on the sorting of 'aa'.  In tsearch2, use COLLATE "C" to
remove an uninteresting dependency on locale sort order (and thereby
allow removal of a variant expected-file).

Also, in citext, avoid assuming that lower('I') = 'i'.  This isn't relevant
to Danish but it does fail in Turkish.