Recent changes to this test has doubled the time run_unittests takes for a windows variant. We should try to make this test shorter, since it currently tests 320,000 interval combinations.

I recommend changing the bitset size from 10 to 7. n=7 tests all of the useful cases than n=10 does, and eliminates redundant tests, like the following:

[1, 2] U [5, 6] = [1, 2] U [5, 6]
[1, 2] U [7, 8] = [1, 2] U [7, 8]

These essentially test the same thing, that we don't accidentally merge two intervals that don't overlap. Testing both is unnecessary.

n=7 reduces the combinations tested from 320,000 to 77,000.

I also think we could add a move iterator here, and a std::move here.