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You can use TestKit's

GradleRunner

to test your extension but it would be likely classified as functional or integration test) There's no

SettingsBuilder

afaik unlike

ProjectBuilder

for project-level testing.

following up in case anyone finds this thread later. The problem is that the gradle testkit wrote an initial line into the settings.gradle file:

rootProject.name = "root"

Ended up needing to write the entire settings.gradle file myself:

settingsFile().writeText(
...