tests: update PHPUnit version

[SMillerDev]

Update to the latest PHPUnit version

[tzi]

Thanks for the PR.

The intent here is still useful: updating the test stack away from the old PHPUnit 4 setup was necessary.

At this point though, this PR is stale relative to the current state of the branch:

• the repository has already moved forward on PHP compatibility and test modernization
• the exact version targets in this PR (PHP >= 7.1, phpunit ^9.5, Travis changes) no longer match the direction we are taking now
• CI is also being migrated away from the old Travis-based setup

So I’m not planning to merge this PR as-is, but the contribution was still helpful in pushing the project in the right direction.

I’m going to close this as superseded by newer work on the branch.