Disable FileSystem static cache in S3 tests

FileSystem has a static cache meaning when the authentication happens in the org.apache.sqoop.testutil.S3TestUtils#setS3CredentialsInHadoopConf method Sqoop import will get the same FileSystem object from the cache thus its authentication via the -Dfs.s3a.access.key and -Dfs.s3a.secret.key properties is effectless. See org.apache.hadoop.fs.FileSystem#get(java.net.URI, org.apache.hadoop.conf.Configuration). 

This static cache should be disabled (by settin fs.s3a.impl.disable.cache to true) in the setup phase of the S3 tests to make sure Sqoop relies on the S3 credentials set via the -D properties.