IMPALA-3017: avoid excessive mem usage for test_very_large_strings
IMPALA-3017: avoid excessive mem usage for test_very_large_strings

Only run the test for parquet (since it doesn't really depend on the
file format) and only execute it serially, since it's very memory hungry.

Change-Id: I0b0ade840ff510fff0fc83532f12b835b3eca76a
Reviewed-on: http://gerrit.cloudera.org:8080/2206
Reviewed-by: Tim Armstrong <2f8e34250f03a653da3dc27f7ec203eafdde80fc@cloudera.com>
Tested-by: Internal Jenkins

cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala