how to remove test cap in drain line

I have a nasty situation. I have a test cap glued into my 3" house drain line just outside the foundation wall, and wonder if anyone can suggest a way to remove it without digging out the line. For example, a roto-rooter style bit that would make a clean 3" circle cut. The attached diagram that illustrate the situation.

=== The back story:

I am building a retirement house in a location in rural Washington state. Plumbing is one of the trades I know the least about. So 5 years ago, when the foundation walls had been poured, but before I framed the house, I called upon a neighbor there for help hooking my house drain line up to my septic tanks. The neighbor is a retired plumber from the US Navy. His advice (he charged me $50 for an afternoon coaching me) was that when the time comes for inspection -- when the drain/waste/vent system needs to be filled with water -- testing is made much easier if you've glued a test cap into the drain line. It avoids the mess of using a test balloon. He said no problem to knock the test cap out later. He'd come out and show me how. After I had glued in the test cap, I got nervous how I would get the darn thing out, so I added a cleanout just upstream of it ... giving me some (limited) access. (see diagram)

Five years later now, I have passed all of the framing/electrical/plumbing rough-in inspections, and I'm ready to install interior plumbing fixtures. But there's still this test cap in my drain line. My neighbor just shrugs about getting it out and says "Aw, it's easy". His preposterous suggestion is just to reach down with pliers and pull it out. But of course there is no way to fit a hand with pliers into a 3" drain line. Is he remembering commercial jobs that had larger diameter lines? Or did he play a Navy practical joke on me? Too late I have learned that he's not a guy I should have trusted.

Reporting back on how I resolved this situation -- for better or worse ....

I did not dig the pipe up. I was able to ram a length of (stiff) 1 inch PEX pipe down the cleanout from 5 feet above. I could feel it pop out the test cap, but I had no way to see whether it did a clean job of it. So I ran water down the drain line to flush out the pieces, and then looked in the septic tank (which still contains only clear water), and I could see two large fragments of test cap, but not all of it. Then I ran a powered snake into the 3" line and let it ream away at the edges of the test cap area for a while. Only a few tiny fragments of test cap came down into the septic tank. Finally, I borrowed a camera (a Milwaukee M-spect) and by snaking it down through the outside 2-way cleanout, I could examine most -- but not all -- of the rim around the test cap. I couldn't see the upper part of the pipe, but I could see that the lower 2/3 of the test cap had been cleanly removed.

I'm gonna start using the system and see if this is good enough. The test cap left its rim behind of course -- about a 1/16" ledge that breaks the smooth inner surface of the pipe. I hoping that won't be enough resistance to cause trouble. Will report back here if it turns out in the end that I am forced to dig the darn thing up anyway.