The test pilot is the pilot episode of 101 Dalmatian Street. Originally produced in 2017, but was never released to the public eye.
Apparently we were going to meet the family through Dylan who was going to introduce us to each of the main characters. According to images that were presented, something was going to happen in the home of the family that deserved to give Dolly a bubble bath. The pilot is very doubtful to appear in the public light, unless Disney says something different.
Some of this pilot can be seen in the Annecy Animation Festival Reel in 2018. These scenes feature Dolly riding her skateboard through Trafalgar Square at Christmas time, being chased by the other pups; Dolly bungee jumping from Big Ben, as well as the announcement image of Dylan with the other dalmatians in motion.
This pilot (like the 1996 101 Dalmatians: The Series pilot) is considered to be lost media as of now.
Differences from the Pilot to the series
Dylan seems much older in this episode and Dolly is very similar to Cadpig (a character from the predecessor series 101 Dalmatians: The Series from 1997).
The style of the animation was similar to the cartoons of 1990s, rather then a more modern style.
One of the scenes from the pilot, shown as part of the Annecy Animation Festival Reel in 2018, appears to be when Dylan gets spooked by a spider in "Walkies on the Wild Side". While the animation looks similar, some differences include Dylan having seemingly painted a bandit mask across his eyes, as well as the spider having a different pattern on them.
According to Storyboards for the Pilot, Dolly was originally planned to be called "Daisy", too, whether the name change was made before or after production of the Pilot is yet unknown.