Minimize + Centralize empty array declarations

[stevenaw]

Fixes #4041

This PR introduces a helper to cache empty array declarations. It will use Array.Empty<T> if possible, falling back to a custom implementation when that's not available.

This should be ported to v4 unless we decide to drop support for net45 there.

[manfred-brands]

The idea and changes are good.
You forgot one in framework directory in DefaultSuiteBuilder.cs: IFixtureBuilder[] attrs = new IFixtureBuilder[0];
I assume you deliberately ignored zero array allocations in the testdata and tests directories.

[stevenaw]

Thanks @manfred-brands ! I've pushed an update to add the missed opportunity in DefaultSuiteBuilder
That's right, I had tried to focus just on distributable code since this was specific to v3.13 (and I thought it might be helpful to keep the branches as in-sync as possible) but I'm happy to include the test projects in this PR as well. Thoughts or preferences?

[manfred-brands]

I would not do more here and for main move to Array.Empty once we have a minimum target of net462

[mikkelbu]

Only one comment, but I'm also happy to merge it as it is