Fix some assertions for integ tests

Signed-off-by: Henry Wang <henwang@amazon.com>

integration/truncindex_test.go

@@ -42,7 +42,7 @@ func TestTruncIndex(t *testing.T) {
 	t.Logf("Get image status by truncindex, truncID: %s", imgTruncID)
 	res, err := imageService.ImageStatus(&runtimeapi.ImageSpec{Image: imgTruncID})
 	require.NoError(t, err)
-	require.NotEqual(t, nil, res)
+	require.NotNil(t, res)
 	assert.Equal(t, imgID, res.Id)
 
 	// TODO(yanxuean): for failure test case where there are two images with the same truncindex.