[INTERNAL] sap.ui.test.TestUtils: Use merge for deep copying
[INTERNAL] sap.ui.test.TestUtils: Use merge for deep copying

PS1: Replace Object.assign() with merge() so that changing something in
	the response doesn't change the fixture

Change-Id: Ib491f38c3643a776885d596b55969ab5d8b7e67c
JIRA: CPOUI5MODELS-256

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