Fix test for TestDumpJSONRefCount

```equals(t, 0, refcount(jv))``` is  unnecessary in new test method,
because  internal pointer variable is already released by ```freeJv(jv)``` when decreases to 0,
so an indefinite value is returned this.
https://github.com/mgood/go-jq/blob/master/jq_test.go#L304

i think that released check by series of process(```equals(t, 1, refcount(jv))``` and ```freeJv(jv)```).