`zdd_change` tests fail on Mac

[ssoelvsten]

As #744 was merged, two unit tests seem to fail for Adiar; but, it only fails on mac machines.

List of failures:
 (*) adiar/zdd/change.cpp zdd_change(A, begin, end) flips root for [1] on (0):
/Users/runner/work/adiar/adiar/tests/adiar/zdd/change.test.cpp:819: Expected: equal to arc: (x2, 0) - -> (x3, 0)
Actual: arc: (x2, 0) ---> (x3, 0)

 (*) adiar/zdd/change.cpp zdd_change(A, begin, end) flips and adds a node for [1] on (1):
/Users/runner/work/adiar/adiar/tests/adiar/zdd/change.test.cpp:967: Expected: equal to arc: (x2, 0) - -> (x3, 0)
Actual: arc: (x2, 0) ---> (x3, 0)

That is, somehow the is_high flag is set to the wrong value. Maybe the wrong case in the policy is used and so the low and high children are swapped? That could be due to the changes in the level merger messing up whether a level is a hit or not. That is, the following commits are probably the origin of the issue:

• 87080a2
• 1efb1c9