Add MockProcessorContext to public test-utils

With KIP-247, we added public test-utils artifact with a TopologyTestDriver class. Using the test driver for a single Processor/Transformer/ValueTransformer it's required to specify a whole topology with source and sink and plus the Processor/Transformer/ValueTransformer into it.

For unit testing, it might be more convenient to have a MockProcessorContext, that can be used to test the Processor/Transformer/ValueTransformer in isolation. Ie, the test itself creates new Processor/Transformer/ValueTransformer object and calls init() manually passing in the MockProcessorContext.

This is a public API change and requires a KIP: https://cwiki.apache.org/confluence/display/KAFKA/Kafka+Improvement+Proposals