Update three test cases in DEREncoderTestCase to use a common helper method

Take a look at these three test cases in DEREncoderTestCase.

Notice how all three test cases have the same code, they just each use a different String value for the object identifier.

To simplify these test cases, it would be good to introduce a new private method with a body similar to the bodies of the methods above but takes the object identifier that should be used as a parameter.

The three test cases can then be updated to simply call the new helper method with the appropriate string parameter.