Improve WebSocket producerBuilderTest

[cbornet]

Motivation

Better test coverage

Modifications

Trivial changes

Verifying this change

• Make sure that the change passes the CI checks.

This change added tests and can be verified as follows:

• Run AbstractWebSocketHandlerTest::producerBuilderTest

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (yes / no) no
• The public API: (yes / no) no
• The schema: (yes / no / don't know) no
• The default values of configurations: (yes / no) no
• The wire protocol: (yes / no) no
• The rest endpoints: (yes / no) no
• The admin cli options: (yes / no) no
• Anything that affects deployment: (yes / no / don't know) no

Documentation

Check the box below or label this PR directly (if you have committer privilege).

Need to update docs?

• doc-required

  (If you need help on updating docs, create a doc issue)

• no-need-doc

  (Please explain why)

• doc

  (If this PR contains doc changes)

[cbornet]

/pulsarbot rerun-failure-checks