Add tests for new String.Join overloads

[AlexGhiondea]

Once dotnet/coreclr/pull/7621 makes its way over to CoreFx, we need to add tests for the new APIs.

[hughbe]

I'll take this once we update the core CLR dependencies here

[AlexGhiondea]

@hughbe that is great! I assigned the issue to you!

[AlexGhiondea]

@hughbe, it looks like @jamesqo has taken over that work and filed dotnet/coreclr#7942 for it.

[AlexGhiondea]

@jamesqo, @hughbe let me know if any of you are planning to add tests in CoreFx side for this.

[hughbe]

Yeah I agreed with James he'd implement and test

[AlexGhiondea]

@hughbe and @jamesqo thank you! :)

[AlexRadch]

I am working on this PR

[karelz]

@AlexGhiondea isn't it tracking same thing as #16172?

[AlexRadch]

@karelz Yes. It is the same thing

[karelz]

Closing as dupe of #16172.