Is there a way to test the injectors?

Sure, we can help you get it running, no problem. Can I ask if it will run on starting fluid? Also, testing the injectors is easier if you remove them.

Here is a guide to help:

https://www.2carpros.com/articles/how-to-test-a-fuel-injector

Also, when you turn the key on without starting the engine can you hear the fuel pump run for about 5 seconds? I would try unplugging the MAF sensor to see if it starts as well. Please go over this guide so we can continue:

https://www.2carpros.com/articles/car-not-running-advanced

Check out the images (below). Let us know what you find.

Starts up and runs for how long? From the description of it running and the backfire and now it doesn't run I would think the fuel pressure regulator on the fuel rail mat have been damaged by the pressure spike from the backfire.
You say you have been working on it for a week? What have you already done?

Will it run if you have your foot on the throttle a little bit? In other words, can you make the engine idle if your foot is on the throttle? Can you upload a short video of the problem so we can see?

Okay, go to the fuel pressure regulator on the fuel rail. Pull the vacuum line off and look for any raw fuel in the hose, try starting the engine, does any fuel come out of the vacuum port on the regulator? Yes, you have gas there, you can replace the regulator diaphragm to repair it. (Book says you replace the entire rail, but you really just replace the diaphragm these days, most of them come with the instructions for the process).
Nothing comes out then we need to look elsewhere. The normal fuel pressure running should be 40-48 PSI on them so your 40 could be OK but if the regulator was leaking it would act like it was flooding all the time like you are describing.
I would also check the MAP sensor voltage; a backfire may have damaged it as well.

Anything?