MINOR: Change KStreamKstreamOuterJoinTest to use distinct left and ri…

…ght types (#15513)

KStreamKStreamJoin was recently refactored to be type safe with regard to left and right value-types. This PR updates the corresponding test to use different value types instead of the same one to improve test coverage.

Reviewers: Bruno Cadonna <bruno@confluent.io>, Matthias J. Sax <matthias@confluent.io>