Fixes part of #5134: Add core.platform.models tests (#5944)
Fixes part of #5134: Add core.platform.models tests (#5944)

* Added initial tests

* Changed tests so they don't access protected classes

* Fix linter errors and actually get rid of all the protected methods

* Fixed linter error to add blank line

* Addressed comments

* Fixed line too long error

* Addressed comments to use self.swap and get rid of extraneous pragma

* Removed extraneous pylint pragmas

brianrodri/oppia,oppia/oppia,prasanna08/oppia,oppia/oppia,souravbadami/oppia,souravbadami/oppia,brianrodri/oppia,prasanna08/oppia,souravbadami/oppia,kevinlee12/oppia,prasanna08/oppia,souravbadami/oppia,oppia/oppia,brianrodri/oppia,kevinlee12/oppia,brianrodri/oppia,prasanna08/oppia,kevinlee12/oppia,oppia/oppia,souravbadami/oppia,kevinlee12/oppia,prasanna08/oppia,kevinlee12/oppia,brianrodri/oppia,oppia/oppia