pgsql: Add a regression test to verify that the stack depth checker

Log Message:
-----------
Add a regression test to verify that the stack depth checker actually
works (and max_stack_depth is not set too high for the platform).
Inspired by trouble report from Brian Betts.

Modified Files:
--------------
    pgsql/src/test/regress/expected:
        errors.out (r1.49 -> r1.50)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/errors.out.diff?r1=1.49&r2=1.50)
    pgsql/src/test/regress/sql:
        errors.sql (r1.12 -> r1.13)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/errors.sql.diff?r1=1.12&r2=1.13)