Remove Core UI from karma tests (#39805)

src/legacy/core_plugins/tests_bundle/tests_entry_template.js

@@ -68,10 +68,10 @@ fetchMock.post(/\\/api\\/capabilities/, {
   headers: { 'Content-Type': 'application/json' },
 });
 
-// render the core system in a child of the body as the default children of the body
-// in the browser tests are needed for mocha and other test components to work
+// render the core system in a element not attached to the document as the
+// default children of the body in the browser tests are needed for mocha and
+// other test components to work
 const rootDomElement = document.createElement('div');
-document.body.appendChild(rootDomElement)
 
 const coreSystem = new CoreSystem({
   injectedMetadata: {