Mark TestUdfs.test_mem_limits to run serially
Mark TestUdfs.test_mem_limits to run serially

This was causing other tests to fail with process mem limit exceeded.

Change-Id: I1407b0896052aece691c681827994961b09d8103
(cherry picked from commit 2bcc46117f504f50ded724fddf74f24bd829c6c6)
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2003
Reviewed-by: Skye Wanderman-Milne <6d4b168ab637b0a20cc9dbf96abb2537f372f946@cloudera.com>
Tested-by: jenkins

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