Clean unit tests

Various clean and test improvements:
- Fix custom unit tests that was not ran by surefire (tests ShutdownStartManyTimes, testMVertexHashCodeAndEquals and testMEdgeHashCodeAndEquals)
- Remove 4 unused suites (testGraphFileTypeJava, testGraphFileTypeGML, testGraphFileTypeGraphML and testGraphFileTypeGraphSON): those suites were actually testing the TinkerGraph implementation. Related features are (not yet) implemented by MapDBGraph
- Clean code typo and format on related tests files