Expand api ResourceVersioner test

You can define the struct type and the test cases all in a single declaration, which I find to be more succinct (you don't define a type name for something you only use once) and easier to read. This would change to something like:

testCases := []struct {
  desc string
  Object interface{}
  Expected uint64
}{
  { "empty api object", Service{}, 0},
  { "api object with version", Service{JSONBase: JSONBase{ResourceVersion: 1}}, 1},
  ...
}
for _, testCase := range testCases { // run tests }