Changing duration returned from DIFF_IN_USEC to unsigned long
Changing duration returned from DIFF_IN_USEC to unsigned long

In the case that the diff returned from the start and end of the test
overflowed an unsigned int, making the container for the result
an unsigned long. (This is the type returned from the macro anyway).

tarruda/check,tarruda/check,tarruda/check,tarruda/check,tarruda/check