[WIP] Buffer stress tests

[CrossR]

Starting to add some buffer stress tests, to narrow down what is happening in onivim/oni2#296.

What is odd currently is that locally the second test fails, despite being a copy-paste of the first test. As far as I can see, I'm resetting correctly, at least based on the buffer test I'm basing this off.

[CrossR]

Pushing the macro test up by another order of magnitude seems to hang the test (ie up to 1 million lines). The terminal also is not happy with something like that though, so its not a surprise.

Similarly, the jump from 6 million to 10 million doesn't seem happy on the undo. The 6 million one is already pretty slow, and drags the tests out.

I'm intending to add a few more of the tests outlined in the linked issue, and basically see how far I can push them and then drop back and add a high, but reasonable test to check for any regressions. I think then it would be useful to duplicate this on the oni2 side, to check the differences between the raw lib and the GUI.