I have reviewed your issue. This is to inform you that there are expected differences between icons of New Test Set and New Test.
The New Test icon has a single chemistry lab flask/bottle icon with a star next to indicate New and New Test Set icon is still a ''flask'' but has a shadow line to indicate more than one flask and still has a star for a new set of tests. New Test in the Test Plan is an icon of a sheet of paper with lines on it like writing with a pencil or typewriter, AND it has a footprint on it to represent a ''step''. These should all make sense and are a general/standard in various tools we have come to expect in Windows. Star to indicate new, single vs. multiple icons to indicate creating one or many of something, pages of paper for a newly written document, etc.
So it is ' working as designed', but if you still want to have an enhancement request - you can suggest a better icon (but I don't think there is anything
better to represent a ''set'' of lab bottles).
I hope above information clarifies your concern.