Testing Graphics in a Testing Notebook
This is a similar issue to Testing Grids in a Testing Notebook and has the same solution: add a {SameTest -> (Rasterize[#1] == Rasterize[#2] &)}
.
Seems related to the issue raised in Testing equality of graphics.
Digging around using FullForm[]
shows that the difference is a string-valued private tag
`Charting`Private`Tag$[random number]
Therefore, we may manually set that tag number to, say, 1 via a rule
nullRule = x_String /; StringMatchQ[x, "Charting`Private`Tag$*"] ->
"Charting`Private`Tag$1";
then the test runs okay: