pgsql: Increase the statement_timeout value used in the prepared_xacts

Log Message:
-----------
Increase the statement_timeout value used in the prepared_xacts regression
test. We have seen some buildfarm failures that seem to be due to this
limit being unexpectedly exceeded when the machine is under load.

Modified Files:
--------------
pgsql/src/test/regress/sql:
prepared_xacts.sql (r1.1 -> r1.2)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/prepared_xacts.sql?r1=1.1&r2=1.2)
pgsql/src/test/regress/expected:
prepared_xacts.out (r1.4 -> r1.5)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/prepared_xacts.out?r1=1.4&r2=1.5)