chore(test): remove test containers from the DOM
chore(test): remove test containers from the DOM

Before destroying the previous modeler instance, the previous
test container is removed from the DOM to speed up DOM queries.

Related to #149

aakhmerov/bpmn-js-properties-panel,bpmn-io/bpmn-js-properties-panel,bpmn-io/bpmn-js-properties-panel