For programming the 7800 overall, the 7800 Development Wiki is very valuable.
For Assembly programming specifically, the 7800AsmDevKit may provide benefit. Understanding the MARIA chip and idiosyncrasies, among other items, the updated 7800 Software Guide is a great resource. A downloadable *.pdf version of that updated guide can be obtained from this post. Some additional in depth specifics can be referenced under the 7800 Tutorials and Guides section of the Wiki.
Testing under real hardware can be managed by acquiring an MCP Development Kit, which connects via USB port from a computer to the second controller port of the console. More tech details can be read from this post, including no modifications to the 7800 console are needed.
There is also the Mateos flash cart with instructions posted. Another flash cart, called the Cuttle Cart II, has been out of production for many years now, and tends to be very pricey when a previously owned one does appear for sale. In development for quite some time with several beta/test carts out in the wild, is the Harmony Concerto; no ETA on final product or mass availability.
In lieu of the above options, the console BIOS can be updated to DevOS as part of creating a complete dev environment. There is an archive of Eckhard Stolberg's Atari VCS 7800 development system page available with respects to origin. The Atari 7800 w/devOS BIOS working with modern PC's? thread can provide some guidance. Also, see the 7800 Dev System mode question thread.
Heads-up if going the hardware modification route with building a DevOS system, while several visitors of the forum reported having such a system built, the console chosen for the modification may wind up being finicky. PC windows compatibility includes support from Windows 95 thru XP (32-bit), and it requires a parallel port. The second revision DevOS files and documents/instructions can be downloaded here:
The next best thing, going outside of real hardware testing and utilizing software emulation instead, is A7800. It is the closest to exact hardware emulation publicly available. An easy setup guide for Windows has been provided. Being a fork of the MAME project, it contains a pretty extensive debugger, easily launched via the '-debug' command line switch [I.E. 'C:\A7800>a7800 a7800 -debug'].