So that you understand why test lead wire meets your needs where general purpose wires don't:
Wire flexibility is determined by two things:
The insulation aspect is pretty easy to understand - the stiffer the insulation, the stiffer the wire. Test lead wire uses a tough, but very pliable rubber for insulation, so not only is it flexible, it's also heavy so it drapes on the workbench rather than standing up stiffly where it's stressed.
The wire is a little bit more interesting.
Notice that the test lead wire has the property:
If you look at a comparable standard stranded hookup wire you'll find this parameter as:
This means that the test lead wire is composed of 65 individual 36 awg wires, while the appliance wire (which is still pretty flexible, just not limp noodle flexible) only has 16 wires of 30 awg each.
With AWG gauges when you drop the gauge by 3 (ie, from 30 to 33 gauge) the area of a slice of the wire (thin disc) drops by a factor of 2 (which, among other things, means the current carrying capacity drops by 2). This area affects the stiffness similarly, so from 30 gauge to 36 gauge, the wire's flexibility increases by almost 4 times, while the current carrying capacity decreases by 4 times.
Of course, that also means you need over 4 times the number of wires (more than 4x since it's not solid copper) to attain the same 'awg' gauge.
But the flexibility increase is worth the extra cost for many applications.
So when you're looking for wire, keep in mind that this parameter (strands/awg per strand) is important.