[SPARK-4152] [SQL] Avoid data change in CTAS while table already existed

CREATE TABLE t1 (a String);
CREATE TABLE t1 AS SELECT key FROM src; – throw exception
CREATE TABLE if not exists t1 AS SELECT key FROM src; – expect do nothing, actually will overwrite the t1.

This PR actually contains the following change:
1) Fix bug describe above;
2) Whitelist the test case.
3) Disable the file split for small single table file(make input splits based on HDFS block)