What should I buy to test whether my DC power supplies are able to provide their rated power?
To simply test a power supply, all you need is a few power resistors and a voltmeter.

Use Ohm's law to compute the resistance that would draw the full rated load from your power supply.  For example, if you have a power supply rated for 12 V at 2 A, then ideally (12 V)/(2 A) = 6 &Omega; would load it to the limit.  To leave a little margin, use the next higher value resistor, like 6.2 or even 6.8 &Omega;.

Connect the resistor to the power supply, and measure the voltage across it with the voltmeter.  If the voltage is still what it's supposed to be, then everything is working.  If the voltage is too low, then the supply can't handle the current.

Note that these resistors need to be able to dissipate significant power.  In the example above, that would be 24 W.  A large 10 &Omega; or 4.7 &Omega; wire wound resistor with adjustable tap can be useful here.

However, in addition to all that, powering a laptop is more complicated.  Laptop supplies aren't just dumb things that produce a fixed voltage up to some maximum current.  They usually have a communication channel and exchange data with the laptop before going to full power mode.  Some manufacturers deliberately throttle back charging when the power supply isn't one of theirs.