AVRO-2593: Improve test_tether_word_count

[kojiromike]

Jira

• My PR addresses AVRO-2593 and references it in the PR title.
• My PR does not introduce any dependencies.

Tests

• My PR is a refactor of an existing test. It does not change primary functionality.

Commits

• My commits all reference Jira issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• My PR does not add functionality and does not require any changes to documentation.

[kojiromike]

I can't seem to reproduce the JAVA=11 test failure locally. When I run in a Java 11 environment locally the tests pass. :/

[kojiromike]

Will revisit in AVRO-2603.