Creating a specific volt/current "test load"
Your calculations are correct, you would need a 2.38&Omega; resistor that can handle 10.5W of power.

Both resistors you found, however, only have a 10W rating. With the 2.5&Omega; at 5V, you would be producing exactly 10W; I don't recommend operating a power resistor at its maximum rating for any duration of time. Components likely exceed their tolerance to some degree, but there's always a margin of error, so you should avoid operating things at the max rating unless you have good reason to do so. In the case of the 2.4&Omega; resistor, you would actually be running it at 10.4W (\$2.08A * 5V\$), which exceeds its rating. Definitely don't do that.

Going back to the desired load: 5V at 2.1A, 10.5W. There aren't precisely any 2.38&Omega; power resistors, but if you double it you get 4.76&Omega; and 4.7 is a common value among resistors. If you put two 10W 4.7&Omega; resistors in parallel, you'll get 2.35&Omega; total resistance, and have 20W total power handling capability. Note that they are 5% tolerance, so actual resistance will vary.

Edit, per comments:

Note that power rating on resistors is the max amount of power that they can dissipate. How much power they need to dissipate depends on how much current they draw. Remember, current (measured in Amperes) is drawn (and determined by) the load, not the power supply*.

For two equal value resistors in parallel, resistance is halved but power handling is additive. So two 10W, 4.7&Omega; resistors in parallel have a resistance of 2.35&Omega; but a total power handling capability of 20W.