Add setInaccessiblePropertyValue helper method and update docs

[unfulvio-godaddy]

Summary

While adding #220 I forgot one method (for @10up/godaddy-wp_mock these are basically the same as in MWC Tests, and with the addition of this PR we can drop them -- figured these are generally helpful and could be part of WP_Mock directly).

This PR also updates the docs related to AccessInaccessibleClassMembersTrait.

Fixes an unrelated flaky test.

Contributor checklist

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly
• I have added tests to cover changes introduced by this pull request
• All new and existing tests pass

Testing

Reviewer checklist

• Code changes review
• Documentation changes review
• Unit tests pass
• Static analysis passes

[coveralls]

coverage: 65.6% (+0.05%) from 65.554% when pulling 77d3998 on chore/add-set-inaccessible-property-value-helper-method into c176fd8 on trunk.

[nmolham-godaddy]

LVGTM, just added few suggestions to add PHP documentation links, it is not a blocker, I just think it would be a good reference

[unfulvio-godaddy]

thanks for the suggestiosn @nmolham-godaddy - I applied those changes with references to PHP.net docs