Testing the basics: 

d1 = 5 hours, 10 minutes, and 0 seconds
d2 = 1 hour, 2 minutes, and 3 seconds
d3 = 1 hour, 2 minutes, and 3 seconds
d3 = 5 hours, 10 minutes, and 0 seconds
d4 = 0 hours, 0 minutes, and 0 seconds

Testing arithmetic: 

d1 = 106 hours, 46 minutes, and 45 seconds
d2 = 0 hours, 0 minutes, and 0 seconds
d3 = 101 hours, 41 minutes, and 40 seconds
d4 = 49 hours, 50 minutes, and 51 seconds
d2 / d3 = 0.5
d3 / d2 = 2

Testing unusual cases: 

d1 = 1 hour, 0 minutes, and 1 second
d1 = 99 hours, 59 minutes, and 59 seconds
d2 = 0 hours, 0 minutes, and 5 seconds
d3 = 1 hour, 0 minutes, and 0 seconds

Test the randomize function: 

d1 = 11 hours, 46 minutes, and 28 seconds
d1 = 0 hours, 29 minutes, and 0 seconds
d1 = 8 hours, 4 minutes, and 21 seconds
d1 = 23 hours, 12 minutes, and 56 seconds
d1 = 11 hours, 15 minutes, and 32 seconds
d1 = 5 hours, 26 minutes, and 18 seconds
d1 = 0 hours, 9 minutes, and 44 seconds
d1 = 15 hours, 14 minutes, and 38 seconds
d1 = 3 hours, 39 minutes, and 1 second
d1 = 2 hours, 0 minutes, and 43 seconds
Average of 10,000 random durations is: 12 hours, 1 minute, and 34 seconds