pgsql: Extend 'guc' regression test to check manipulations of datestyle

Log Message:
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Extend 'guc' regression test to check manipulations of datestyle as
well as vacuum_cost_delay. Since datestyle is a string variable,
this exercises memory allocation issues that might not appear when
modifying an integer GUC variable. Also, we can observe the side
effects of changing datestyle to check that assign hooks are called
at the right times.

Modified Files:
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pgsql/src/test/regress/sql:
guc.sql (r1.1 -> r1.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/guc.sql.diff?r1=1.1&r2=1.2)
pgsql/src/test/regress/expected:
guc.out (r1.1 -> r1.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/guc.out.diff?r1=1.1&r2=1.2)