Change complex obs handler tests to extend BaseContextSensitiveTest

The complex obs handler tests all utilize PowerMock to create a dummy context. This leads to a decrease in test coverage (see https://issues.openmrs.org/browse/TRUNK-5150). They should be rewritten leveraging Spring Test, subclassing BaseContextMockTest

The following classes need to be changed for this:

• AbstractHandlerTest
• BinaryDataHandlerTest
• BinaryStreamHandlerTest
• ImageHandlerTest
• MediaHandlerTest
• TextHandlerTest