Partitioning a string into chunks
Nice and clean implementation. I tested this with the tests (the new ones) of my implementation and they all passed (skipping the tests with chunkSize == 0).  

The code could be sligthly more readable by having some vertical space to group related code, for instance after validating the input.  

You could improve this a little bit by just having this  

if (chunkSize == 1)
{
    yield return value;
    yield break;
}  


after the validation, in this way you wouldn't need to create a StringBuilder nor having the enumerator.  



A slightly different approach could be to remove the for loop. It makes the intent more clear (IMO) and removes the need to double check return value of enumerator.MoveNext().  

Unfortunately this makes the code 2 lines (3 with the vertical spacing) longer  

        var sb = new StringBuilder(chunkSize);
        var enumerator = StringInfo.GetTextElementEnumerator(value);
        var counter = 0;
        while (enumerator.MoveNext())
        {
            counter++;
            sb.Append(enumerator.GetTextElement());
            if (counter == chunkSize)
            {
                yield return sb.ToString();

                sb.Length = 0;
                counter = 0;
            }
        }
        if (counter &gt; 0)
        {
            yield return sb.ToString();
        }




Regarding naming of unit tests, I usually (not in the posted question of mine) use the pattern   

UnitOfWork_StateUnderTest_ExpectedBehavior  

like shown in the accepted answer over here: unit test naming best practices