Added WP minimum env And Fixed createPost issues in WP<5.8

[iamdharmesh]

Description of the Change

This PR adds WP Minimum env(WP 5.2) in GH actions and fixes below failed commands and tests in 5.2 env.

• createPost
• setPermalinkStructure

Closes #33

Tests are still failing for the below commands in WP minimum env and I have created the GH issues as below:

• createTerm - createTerm tests are failing in WP minimum env.  #35
• openDocumentSettingsPanel & openDocumentSettingsSidebar - openDocumentSettingsPanel & openDocumentSettingsSidebar tests are failing in WP minimum env. #36

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Changelog Entry

Added - WP minimum(5.2) env in GH actions to test with.
Changed - Made createPost and setPermalinkStructure compatible with WP minimum(5.2) env.

Credits

Props @iamdharmesh

[cadic]

@iamdharmesh thank you for fixing this and creating new issues related to other commands!

I can confirm that isolated tests for createPost and setPermalinkStructure commands against 5.2 and latest WordPress run successfully.

LGTM!