circular-buffer: Test overwrite on non-full buffer

[petertseng]

This test is useful to make sure implementations don't drop items from
non-full buffers when using Overwrite.

[petertseng]

Examining other language tracks:
http://x.exercism.io/problems/circular-buffer

Tracks that include this test:

• ruby (the first version of the exercise I did, and I failed this exact test in that track, which is why I noticed this difference when compared to Go's)
• rust (at my recommendation when the exercise was about to be added to that track)

Tracks that do not include this test:

• erlang (but it does test their equivalent of ovewrite on a completely empty buffer, so maybe that is close enough?)
• javascript
• lfe (same comment as erlang)
• lua

So it is possible that the Ruby track was the first to actually explicitly test this behavior of Overwrite (what should it do when the buffer isn't full), and I guess now's a good a time as any to ask whether it should be spread to the other tracks with circular-buffer.

E: Yeah I see, it wasn't in the original version of Ruby circular-buffer but was added later in exercism/ruby@5560b7a for exercism/ruby#123 - so a test for that behavior got added to Ruby track but not ported over to other tracks as well, interesting to see that history.

[petertseng]

Assumption: I should bump testVersion for this (because it might cause previously-passing solutions to fail). I will do so and amend the commit.

E: done so.
E: forgot I need to increment TestVersion in example.go as well, so broke the build. amended again, should be good this time.

[kytrinyx]

Yeah, I think adding the test is a good idea. @soniakeys do you see any gotchas with this?

[soniakeys]

Very nice. Thank you, Peter!