Add multi-php testing to the test matrix.

[desrosj]

Proposed changes

This configures multiple PHP versions to be tested through GitHub Actions until the test runner merges WordPress#212 for native support.

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Video

Checklist

• I have read the CONTRIBUTING doc
• I have viewed my change in a web-browser
• Linting and tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[khalbaaek]

@khlabaaek