Test and style check for pravega samples

[RaulGracia]

Problem description
Currently, there is no testing or checking mechanism to show if samples have been affected by a change in a Pravega release. Moreover, now the repository does not have a style checker as in pravega repository. It would be necessary to introduce both mechanisms, especially considering an increasing number of sample applications.

Problem location
Testing and style checker as a gradle task.

Suggestions for an improvement
Introduce testing and style checker.