Refactor UTXOTest.java and add two new test methods to it

[MethDamon]

Changes:

• Split multiple asserts into test methods
• Added test method for getIndex()
• Added test method for getHash()

Also, this test uses the normal java serialization rather than writeObject() and readObject(), is this done willingly or by mistake?

[MethDamon]

@schildbach Can you take another look at this?

[schildbach]

I think this is very similar to #1813. Splitting the test into many "micro tests" imho doesn't scale.

I see however a couple of new asserts ~~"hidden" in your refactoring~~. If you think they're worth to have, we could add them.