First is the fact that an M&M's diameter is about 13mm, which means you don't get an efficient packing arrangement in a 16mm wide tube. In this case, you are essentially "stacking" M&M's up in the tube.
The second thing that makes this tricky is the volume of the test tube. A 15ml test tube should have a volume of 15,000 cubic millimeters. Given the measurements provided, the volume calculates to 20,106.19 cubic millimeters. So either there is additional available volume on in the tube above the 15ml line, or the measurements are for the outside dimensions rather than the inside. Another thing to consider is that the bottom of test tube is curved which means an M&M will not site flat on the bottom.
Nonetheless, using the formula in the article, a test tube should hold about 16 M&M's. However, I happened to have a test tube very similar to the one mentioned in the question. The inner diameter was 15mm, and it held 14 M&M's up to the 15ml line. This is likely due to the issues noted above. Therefore, a standard 15ml test tube should hold 14 M&M's.