[SPARK-4319][SQL] Enable an ignored test "null count".

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #3185 from ueshin/issues/SPARK-4319 and squashes the following commits:

a44a38e [Takuya UESHIN] Enable an ignored test "null count".
[SPARK-4000][Build] Uploads HiveCompatibilitySuite logs

This is a follow up of #2845. In addition to unit-tests.log files, also upload failure output files generated by `HiveCompatibilitySuite` to Jenkins master. These files can be very helpful to debug Hive compatibility test failures.

/cc pwendell marmbrus

Author: Cheng Lian <lian@databricks.com>

Closes #2993 from liancheng/upload-hive-compat-logs and squashes the following commits:

8e6247f [Cheng Lian] Uploads HiveCompatibilitySuite logs