implement right unit tests

[rahulbats]

• add asserts to tests with souts
• fix https://github.com/newrelic/kafka-connect-newrelic/blob/master/newrelic-kafka-connector/src/test/java/com/newrelic/telemetry/TelemetryMetricsSinkTaskTest.java
• fix https://github.com/newrelic/kafka-connect-newrelic/blob/master/newrelic-kafka-connector/src/test/java/com/newrelic/telemetry/TelemetryEventsSinkTaskTest.java