test(use-colors): change test expectations

Author: MatteoGabriele

test/nuxt/composables/use-colors.spec.ts

@@ -77,8 +77,6 @@ describe('useCssVariables', () => {
     await nextTick()
 
     expect(useMutationObserverMock).not.toHaveBeenCalled()
-    expect(lastMutationObserverInstance).not.toBeNull()
-    expect(lastMutationObserverInstance!.observe).toHaveBeenCalledTimes(1)
 
     wrapper.unmount()
   })
@@ -95,8 +93,7 @@ describe('useCssVariables', () => {
 
     await nextTick()
 
-    expect(lastMutationObserverInstance).not.toBeNull()
-    expect(lastMutationObserverInstance!.observe).toHaveBeenCalledTimes(1)
+    expect(useMutationObserverMock).toHaveBeenCalledOnce()
 
     wrapper.unmount()
   })