In boundary exchange UT, fix interface to F90 exchange routine

In the cpp file we declare and call the interface with only one 1d field, while in the F90 file we declare the subroutine with two 1d fields. Compiler does not complain (in the cpp file declaration and call agree on the signature), and the linker does not complain cause it's an extern C function (so no signature check from the compiler).

We need to fix the cpp file (or change the F90 to accept a single minmax field rather than a min and a max fields).