Add support unit tests for ConcurrentHashSet, #1117

[paulirwin]

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Add support unit tests for ConcurrentHashSet

Fixes #1117

Description

This ports tests from Harmony for Collections.synchronizedSet and ConcurrentHashMap and adapts them to our ConcurrentHashSet.

This also adds nullable reference type checking to ConcurrentHashSet, which exposed some nullability issues. Given that our current use of this type is for values that should never be null (or would otherwise throw an NRE upon use as null is not checked-for), this adds a generic type constraint of notnull to ensure that it cannot be used with nullable reference types. Work would need to be done (under proper unit test coverage) to make this null-friendly if that is needed in the future.

[NightOwl888]

Thanks for the PR.

I have been considering phasing out the SystemTypesHelpers class (or at least most of the extension methods in it), so ideally any new tests we add would not have any dependencies on those extension methods (.add(), .contains(), .size(), etc.).

If we do keep them, they will definitely need a review. Some of the methods perform poorly or have better alternatives (.toString(), removeAll(), retainAll(), and ToByteArray() to name a few). The .append() overloads are generally there to allow culture invariant numbers to be appended, since .NET doesn't have that built-in. However, I am adding optional parameters to allow passing format and IFormatProvider on OpenStringBuilder in J2N. So, we should probably hold off on the review/removal of SystemTypesHelpers until after J2N has a build with OpenStringBuilder where those can be replaced directly by swapping in OpenStringBuilder for StringBuilder.