I have a multi-project build with tests in sub-projects and in a parent project. The build is aggregated so that the parent project runs all tests in child projects.
I configured it so that there's no parallel execution of tests in both the sub-projects and the parent project, via
parallelExecution in Test := false
I think you can apply a setting across projects using scope
parallelExecution in ThisBuild := false
I don't know if you can combine that with scope
Test, but it might not be necessary.