When you have a limited amount of nuclear material, it is perfectly reasonable to run the "tests" in a combat environment. What do you have to lose? If it fails to go boom, it would have done so in the test environment too. If it does go boom, mission accomplished.
I suspect Saddam's definition of a "combat environment" will be somewhere in Israel or a major U.S. city. He won't do it personally. He will let his Al Qaida buddies attend to that task while he watches the homefront.
The so called "Little Boy" atomic bomb the US dropped on Japan had never been tested. It worked fine. The "Fat Man" type was tested, once, the second bomb of that type was dropped on Japan. Both the test and the "live" drop, worked fine. So I"d say the odds were pretty good. Saddam's designers have access to computers that would have made the Manhatten Project designers shake their heads in disblief. So do you for that matter, but likely what Saddam has is step up from your average PC, maybe a couple of them. Would they get the best possible yield in the smallest possible package? Probably not, but they wouldn't be likely to get a fizzle either.
Historically, 50%. The Trinity test in July 1945 was of the implosion bomb (what was to become Fat Man), while the first gun bomb (Little Boy) exploded was over Hiroshima. The scientists knew that the gun bomb would work as advertised, while they didn't know whether the implosion bomb would.
Morever, even if the first Iraqi bomb doesn't reach critical mass, what'll be left is a dirty radiological bomb, which would serve the Islamists/Saddam's purposes just as well. If that first explosion doesn't happen in enemy territory, Saddam knows that UN or no UN, Iraq becomes a self-lighted glass parking lot.
What if one (or more) of the Pakistani tests back several years ago was an Iraqi nuke? You don't suppose that the Iraqi nuke people and the Paki nuke people might have talked? Nahhh! Way to outlandish!
That's what the U.S. did with the atomic bomb dropped on Hiroshima ("gun" design, using U-235). The scientists had such confidence it would work, plus the U-235 was so expensive and time-consuming to produce, that they did not test the design before using the bomb. [It's presumably even easier now, since the Iraqis wouldn't be working from scratch.]
On the other hand, the much more complicated plutonium-based implosion-type design was tested before it was dropped on Nagasaki.
All the talk about a hypothetical Iraqi atomic bomb seems to be of the U-235 gun design. (North Korea, on the other hand, first focused on the plutonium implosion type, and the North Koreans only switched to U-235 after the agreement in the 1990's that they have now admitted abrogating.)