KAFKA-16001: Migrated ConsumerNetworkThreadTest away from ConsumerTes…

…tBuilder (#16140)

Completely migrates ConsumerNetworkThreadTest away from ConsumerTestBuilder and removes all usages of spy objects and replaced with mocks. Removes testEnsureMetadataUpdateOnPoll() since it was doing integration testing. Also I adds new tests to get more complete test coverage of ConsumerNetworkThread.

Reviewers: Kirk True <[email protected]>, Lianet Magrans <[email protected]>, Philip Nee <[email protected]>, Matthias J. Sax <[email protected]>