While you are in all this testing mode you can test any audio device to see what range of input voltages it works over and if and when it introduces distortion.
I have gone on about it before but doing level calibrations in and out of your interfaces is also an interesting thing to do. In a previous system I had three sound cards running at once and I discovered that one of them was inverting all of its 8 outputs for some reason.
Another good thing to do is to see what things like your Pro Channel modules are doing to the signal and how they are distorting things
(on purpose of course) The Console emulator is another process which will show up some interesting things in terms of adding harmonics and things to input signals and distorting waveforms.
You certainly can use your DAW as an audio testing station. Many DAW's have test tone signal generators as
Jim suggests and there are free and paid ones as well that can generate a whole lot of stuff. There are level meters as well. But this is where you need to calibrate your audio interface so you know what the actual voltage is that appears on any output for any given level. You could generate test signals and route them to an output on your interface. From there over to your unit under test and back through an input again. You could run the spectrum analyser and a nice CRO plugin on a track and monitor the input signal that way. You would be able to run everything in real time. And make recordings too of course of any testing procedure.
You can also use your DAW test setup to check polarity of every speaker and microphone you have. You create an signal that is not symmetrical. ie the top half of the signal is different to the bottom half. You can setup a microphone into a CRO now and run the test signal out to every speaker in your studio
(one at a time of course) The mic is placed in front of the speaker and you will see if the speaker is moving the correct way or not. The CRO will display the test signal either the right or wrong way up. I have a bunch of mono speakers placed around my studio to assist me with monitoring in other parts of my room. I found some of those were wired the wrong way around.
Once a speaker is calibrated you can test every mic you own to make sure every diaphragm is moving the right way as well. I found a few mics out of several that were wired incorrectly displaying the waveform the wrong way around. I was using one of them for high hats at one stage.
(that sound would have been wrong and hard to mix)