Declare some missing dynamic properties in tests

[jamisonbryant]

Adds the required trait to suppress deprecation warnings related to dynamic props in the plugin test suite.

I figured the trait was the best solution since these are just test cases, however if it would be better for me to just declare the props explicitly (& well-typed) I'm happy to do that instead.

[ADmad]

It would be better to declare the properties used instead.

[LordSimal]

Make sure to run composer cs-fix to auto-fix CS problems.
But in the end composer cs-check should all be green before you commit.

[markstory]

Thank you 🎉