SQLite: Support custom wp-content directory

[swissspidy]

Some other commands' tests have tests using a custom wp-content directory. This change ensures those tests pass when running with SQLite.

Adds test coverage.

[schlessera]

@swissspidy Wouldn't it be more correct to actually activate the SQLite plugin instead? This way we wouldn't need custom hard-coded logic like the above, and we'd get any future extra steps for free. Is there a reason why we're not activating the plugin right now?

[swissspidy]

I thought about that, but:

• The drop-in needs to work even before install and before the plugin is activated
• The drop-in actually automatically activates the plugin if it's not yet active. See source
• This change here is still needed, because the drop-in has a hardcoded path to the main plugin file. So if you change the location of WP_PLUGIN_DIR, that string needs to be updated, which is what this PR achieves.