paralllelize unittest

[zhouqingqing]

1. we shall reduce unittest time by parallize the many unittest - currently they are not using all cores. Be careful, tpcds/h are not parallizable because they both have 'customer' table and we don't support database or scheme to separate them.

2. clean up the tests along the way. We have several ways to write results comparison, and use TU.ExecuteSQL(sql, results) shall be the ideal form for many of them if we only care about results. Example:

sql = "select * from (select a1*a2 a12, a1 a6 from a) b;"; TU.ExecuteSQL(sql, "0,0;2,1;6,2");