[libcxx][test] MoveOnly's comparisons are non-member

... so that comparisons with an `int` LHS and `MoveOnly` RHS are valid, as is necessary for the `partial_sort_copy` test to pass with an implementation that doesn't force a conversion to the type of the RHS as libc++ does.

GitOrigin-RevId: ad4a6ce10c43d65dd12502bda26dbc983f9bef4e
1 file changed