How to disable a module dependents module? Not the module but the feature
If you mean 


  Can I disable/uninstall the BEF module without first disabling/uninstalling the BEF Test Content module (or any other modules that depend on it)


Then no, that's not possible. 

"Dependency" means that one module cannot function without the other. In this case, the BEF Test Content module will be unable to function unless the BEF module is also installed and enabled.

Drupal understands this constraint, and makes sure the you can't break your site by removing functionality from one module that is required by another.

If you mean


  Can I disable/uninstall the BEF Test Content module without disabling/uninstalling the BEF module


Then yes, you can either disable/uninstall it through the UI as normal, or use Drush. BEF doesn't have a dependency on BEF Test Content, so Drupal won't stop you from removing it.

The only caveat to that is if there are other constraints introduced by the BEF Test Content module. For example, if it provides a field type which is in use, the module cannot be disabled until the field is detached from all entities. But the admin UI will tell you explicitly if that's the case