test: Update Popover test to check for className support

public/app/core/components/Tooltip/Popover.jest.tsx

@@ -6,7 +6,7 @@ describe('Popover', () => {
   it('renders correctly', () => {
     const tree = renderer
       .create(
-        <Popover placement="auto" content="Popover text">
+        <Popover className="test-class" placement="auto" content="Popover text">
           <button>Button with Popover</button>
         </Popover>
       )

public/app/core/components/Tooltip/__snapshots__/Popover.jest.tsx.snap

@@ -2,7 +2,7 @@
 
 exports[`Popover renders correctly 1`] = `
 <div
-  className="popper__manager"
+  className="popper__manager test-class"
 >
   <div
     className="popper__target"