[SPARK-35693][SS][TEST] Add plan check for stream-stream join unit test

[c21]

What changes were proposed in this pull request?

The changed unit test was introduce in #21587, to fix the planner side of thing for stream-stream join. Ideally check the query result should catch the bug, but it would be better to add plan check to make the purpose of unit test more clearly and catch future bug from planner change.

Why are the changes needed?

Improve unit test.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Changed test itself.

[c21]

cc @cloud-fan (original author) if you have time to take a look, thanks.

[c21]

Close & Reopen PR to trigger unit test on github action again.

[c21]

Rebased to latest master.

[SparkQA]

Kubernetes integration test unable to build dist.

exiting with code: 1
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/44075/

[SparkQA]

Test build #139549 has finished for PR 32836 at commit cf9b7b9.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/44086/

[SparkQA]

Test build #139559 has finished for PR 32836 at commit 534459b.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[cloud-fan]

thanks, merging to master!

[SparkQA]

Kubernetes integration test status failure
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/44086/

[c21]

Thank you @cloud-fan for review!