Add tests for foreign keys with case-insensitive collations

Add tests for foreign keys with case-insensitive collations

Some of the behaviors of the different referential actions, such as
the difference between NO ACTION and RESTRICT are best illustrated
using a case-insensitive collation.  So add some tests for that.

(What is actually being tested here is the behavior with values that
are "distinct" (binary different) but compare as equal.  Another way
to do that would be with positive and negative zeroes with float
types.  But this way seems nicer and more flexible.)

Discussion: https://www.postgresql.org/message-id/ea5b2777-266a-46fa-852f-6fca6ec480ad@eisentraut.org