[INTERNAL][FIX]sap.ui.dt: Safeguard for MutationObserver
[INTERNAL][FIX]sap.ui.dt: Safeguard for MutationObserver

Some tests were failing in Karma because a mutation target could not be
retrieved, causing the check for the shadow root host to crash.

Change-Id: I38a832d2f614112eb93bdc83d265816eddbdb742
BCP: 2280163191

SAP/openui5,SAP/openui5,SAP/openui5,SAP/openui5