IMPALA-3954: Add unique_database to scanner test
IMPALA-3954: Add unique_database to scanner test

Without a unique_database hdfs-text-scan-with-header.test will create a
table "functional.mixed" and parallel executions of the test will
interfere with each other. This change adds a unique_database to the
test call to prevent this.

Change-Id: Ic48a4bac3df6a40cb5cb10c6f1c42583952c6c86
Reviewed-on: http://gerrit.cloudera.org:8080/4045
Reviewed-by: Michael Brown <97e4d5d0e400e5ad37382425ea95bde8b3933403@cloudera.com>
Reviewed-by: Tim Armstrong <2f8e34250f03a653da3dc27f7ec203eafdde80fc@cloudera.com>
Tested-by: Internal Jenkins

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