Enable decimal tests on all table formats

In CDH4 we didn't support decimal tests (e.g. tests using decimal_tbl and decimal_tiny) for formats that Impala couldn't write because Hive didn't support writing DECIMAL. In CDH5, we should be able to run decimal tests on other formats because Hive 0.13 supports writing DECIMALs.

I have a patch prepared to enable these tests, but tests won't work yet due to https://issues.apache.org/jira/browse/HIVE-7373 – 0 is NULL in Hive (e.g. cast(0.0 as decimal(1,1) is NULL), and our test tables have 0s.

When HIVE-7373 is resolved, we should enable the decimal tests.