Port and add more tests

There are many Java tests that we do not yet have in the Rust version. You can find most of them in [this file](https://github.com/jonhoo/flurry/blob/master/jsr166/test/ConcurrentHashMapTest.java) from JSR166 and in [this folder](https://hg.openjdk.java.net/jdk/jdk13/file/tip/test/jdk/java/util/concurrent/ConcurrentHashMap) from JDK. Let's port all of them and get that [test coverage](https://codecov.io/gh/jonhoo/flurry) up! I would particularly love to get some of the concurrent tests ported over!

There are also a good chunk of tests over in [`hashbrown`](https://github.com/rust-lang/hashbrown/tree/master/tests) that would be good to also run against flurry.