Within the `PostFactory` class, ensure that when using the `withTags`
method and creating tag terms that an existing term doesn't already
exist for a given name before trying to create it.
With the previous implementation, there would be multiple terms if the
PostFactory was used multiple times with the same tag name.
Given that `PostFactory` now has a dependency on `EntityTypeManger`,
this has been added as a service within `opdavies_blog_test` and needs
to be resolved from the container before trying to use it within a test.
This commit also updates the usages in `PostTest` so that those tests
continue to work and pass.
References #3