[master] Update test_mysql.py

[anonymousdouble]

What does this PR do?

refactor with chain constant value assignment to make code more Pythonic, concise and efficient.

What issues does this PR fix or reference?

Fixes: No

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

Yes/No

Please review Salt's Contributing Guide for best practices.

See GitHub's page on GPG signing for more information about signing commits with GPG.

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