int4->bool test coverage

I was surprised to learn that 2 is a valid boolean (thanks Berge):

# select 2::boolean;
 bool
──────
 t

... while '2' is not:

# select '2'::boolean;
ERROR:  22P02: invalid input syntax for type boolean: "2"
LINE 1: select '2'::boolean;
               ^
LOCATION:  boolin, bool.c:151


The first cast is the int4_bool function, but it isn't covered by the
regression tests at all. The attached patch adds tests.

Christoph