I am new to soldering. I've soldered the standoff leads onto my Arduino Nano 33 IoT, and now I want to run a sketch that somehow tests all the pins. For the data pins, I see an easy way:
I don't think I understand the question. What do you mean by "other pins"? What exactly do you want to test?
The Nano 33 IoT is different from the classic Nano in that it has native USB capabilities, so pins 0 and 1 are not used for communication with the computer. This means you can use those pins for anything you like without interfering with communications with the computer. So you can just use RX and TX as digital pins 0 and 1.
Here's a sketch I wrote for testing pins on Arduino boards. It can be used to check digitalWrite(), digitalRead(), analogWrite(), analogRead(), and interrupts: