Scala: Package test from multiple sub projects

Question

There is a very large Scala project with many sub projects, assume there are 4 sub projects only: project-a, project-b, project-c, project-d.

Project structure is standard, something like:

$ tree project-a
├── project-a.sbt
├── src
│   ├── main
│   │   ├── resources
│   │   └── scala
│   └── test
│       ├── resources
│       └── scala

I want to package the tests of sub projects into a single jar file so that I can run the tests later.

1. How to package tests of all sub projects into a single jar?
2. How to package tests of some sub projects into a single jar? For example: project-a, project-b only?

Thanks and appreciate all the supports.