MINOR: Add test demonstrating re-use of KGroupedStream with Optimizations enabled

[bbejeck]

Right now if a repartition is required and users choose to name the repartition topic for an aggregation
i.e kGroupedStream = builder.<String, String>stream("topic").selectKey((k, v) -> k).groupByKey(Grouped.as("grouping")); The resulting KGroupedStream can't be reused
with optimizations are disabled, as Streams will attempt to create two repartiton topics with the same name.

However, if optimizations are enabled then the resulting KGroupedStream can be re-used
For example the following will work if optimizations are enabled.

kGroupedStream = builder.<String, String>stream("topic")
                             .selectKey((k, v) -> k)
                             .groupByKey(Grouped.as("grouping"));

kGroupedStream.windowedBy(TimeWindows.of(Duration.ofMillis(10L))).count();
kGroupedStream.windowedBy(TimeWindows.of(Duration.ofMillis(30L))).count();

This PR provides a unit test proving as much.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[bbejeck]

ping @guozhangwang, @mjsax, and @vvcephei for reviews

[mjsax]

LGTM