Try to make tests ignored after PHOENIX-4981 pass again

After porting PHOENIX-4981 to 4.x-cdh an addendum commit was made to backport changes to Spark 1.6 ( version shipped with cdh 5.15 ) . After changes were made a few tests started failing and were ignored in the commit.

AggregateIT.testExpressionInGroupBy
AggregateIT.testGroupByCase
AggregateIT.testGroupByDescColumnWithNullsLastBug3452

Following three testcases are using syntax not available in Spark 1.6 hence will be permanently ignored.

OrderByIT.testDescMultiOrderByExpr
OrderByIT.testNullsLastWithDesc
OrderByIT.testOrderByReverseOptimizationWithNullsLast