pgsql: Add a delay at the start of the stats test, to let any prior

Log Message:
-----------
Add a delay at the start of the stats test, to let any prior stats
activity quiesce.  Possibly this will fix the large increase in
non-reproducible stats test failures we've noted since turning on
stats_row_level by default.

Modified Files:
--------------
    pgsql/src/test/regress/expected:
        stats.out (r1.7 -> r1.8)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/stats.out.diff?r1=1.7&r2=1.8)
    pgsql/src/test/regress/sql:
        stats.sql (r1.5 -> r1.6)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/stats.sql.diff?r1=1.5&r2=1.6)