Why will the 777X be tested for crosswind handling with wingtips raised?
If you read the details in the Federal Register, you can see that this refers to crosswind handling on the ground, not in the air:

The folding wingtips and their operating mechanism must be designed
for 65 knot, horizontal, ground-gust conditions in any direction as
specified in § 25.415(a). Relevant design conditions must be defined
using combinations of steady wind and taxi speeds determined by
rational analysis utilizing airport wind data.

And:

The airplane must demonstrate acceptable handling qualities during
rollout in a crosswind environment, as wingtips transition from the
flight-deployed to folded position, as well as during the unlikely
event of asymmetric wingtip folding.