Add Java tests to the CI and fix buggy Java output

[JasonGross]

We should test the Java code. I'm not sure where to find Java tests, though. cc @peterdettman @dghgit,

[peterdettman]

Hi Jason, happy to lend a hand here, but it may be a week or two.

[jvz]

Here's a fairly simple combination of operations which give different results in Java and C: https://gist.github.com/jvz/0bd0ff04b47524da8b8aedc16e79a4eb

I've seen similar discrepancies in a few other operations similarly.

[spitters]

This commit by @eorloff might solve this issue:
https://github.com/eorloff/fiat-crypto