Hey Sandeep,
Wherever there is software development, there would be requirement of testing as well. There is no doubt in the scope of testing, it would be there. Because there is no relevant certification such as a Hadoop Tester in the market yet, there would be people who may think that there is no scope for testing. But where there is development of software, there has to be a testing team.
Knowledge of HDFS and MapReduce framework is absolutely necessary for being good in the field of Big Data testing. The programs majorly are written in Java, along with a few scripting languages like Python and Ruby and higher level languages like Pig and Hive. A good understanding about the framework would result in better chances to excel in Big Data testing. If you are interested, I would recommend you the course "Hadoop Certification Made Easy".
Hope this helps!
Best,
Nitesh