test-sizeof: show that a small 64 field is not enough to force the enum to be 64...

test-sizeof: show that a small 64 field is not enough to force the enum to be 64 bits

On both 32 and 64 bits, the result is:
enum Enum → 32 bits, unsigned
enum BigEnum → 32 bits, unsigned
enum BigEnum2 → 64 bits, unsigned
big_enum2_pos → 4
big_enum2_neg → 8

The last two lines show that even if the enum is 64 bit, and the field of an
enum is defined with UINT64_C(), the field can still be smaller.