Add test for BSC PR 265

[quark17]

There's a test attached to BSC PR 265. We need to decide where to put it -- make a directory for performance tests, such as bsc.bluesim/perf/? or else just a directory for tests associated with GitHub PRs and issues, such as bsc.github/265/, along the lines of bsc.bugs/? And and we need to decide how to write the test procedure -- can we use timeout to detect when the test runs too long, or? The test file has a parameter N that might need to be further increased to N=4096 to guarantee that the efficiency improvement is being observed (and not just running on a sufficiently fast computer).

[bpfoley]

Yeah, this is an interesting one. I guess it depends how sensitive we want our timing tests to be, and what sorts of performance regressions we want to catch.

Since this a small test and is dramatically slower with the O(n^2) performance bug, I'd say we can be pretty brute force. Even when running slowly, it only runs for ~30s, I think it would be fine to put it in the ordinary test suite for now.

I'm guessing that having a timeout ~2-3x whatever the typical runtime is on your machine might a sensitive enough threshold that will catch the buggy O(n^2) behavior, but won't be so sensitive that it will fire when run on a slow machine.