IMPALA-4745: fix TestScratchLimit failure on S3
IMPALA-4745: fix TestScratchLimit failure on S3

The commit "IMPALA-3202,IMPALA-2079: rework scratch file I/O"
improved efficiency of scratch file use in some scenarios.
TestScratchLimit::test_with_low_scratch_limit started failing
on S3, because it expects to use more than 50MB of scratch space.

Testing:
Ran the test in a loop locally for 50+ iterations - didn't see
any failures.

Change-Id: I607b4c6ad10eba0e6c7bc8d6e640d42da26ee6c8
Reviewed-on: http://gerrit.cloudera.org:8080/5654
Reviewed-by: Tim Armstrong <2f8e34250f03a653da3dc27f7ec203eafdde80fc@cloudera.com>
Tested-by: Impala Public Jenkins

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