Is it possible to have multiple source roots in unit tests?

You can simply add test sources directory programmatically in the specific test

VirtualFile testRootDir = myFixture.getTempDirFixture().findOrCreateDir("testRoot");
PsiTestUtil.addSourceRoot(getModule(), testRootDir, true);
Disposer.register(myFixture.getTestRootDisposable(), () -> PsiTestUtil.removeSourceRoot(getModule(), testRootDir));

Or provide custom LightProjectDescriptor and override configureModule()

Thanks Yann Cebron for the hints!

I aim to do this setup for all tests, so the LightProjectDescriptor will be the way to go.

There is one catch though. I wanted the new SpockLightProjectDescriptor to be a standalone class, so I added the fixture as a constructor parameter.

However, there is a circular dependency now

1. The SpockLightProjectDescriptor needs myFixture but myFixture is null until the LightProjectDescriptor.setup() is called. 
2. The LightProjectDescriptor.setup() calls getProjectDescriptor().

I can workaround it of course, but I was wondering if I could do all the stuff in the SpockLightProjectDescriptor without the fixture.

1. Creating a virtual folder and adding it to source roots is easy.
2. Getting hold of myFixture.testRootDisposable would mean calling an internal API (module.project as ProjectEx).earlyDisposable).

When I commented out the suggested Disposer.register(myFixture.getTestRootDisposable(), () -> PsiTestUtil.removeSourceRoot(getModule(), testRootDir)); code, I thought that meant the source root wouldn't be removed after every test. However, each test seems to start fresh with only the temp://src directory setup.

There is another problem. If I follow your steps and create src/main/java , the action is available even though it's not in a real IntelliJ instance.

When debugging, the CreateTemplateInPackageAction#isAvailable() returns true in tests whereas in real instances it returns false when the action is run in the src/main/java directory.

Debugging more, the difference can be spotted in the attached pictures:

1. Both in IntelliJ and the test the fileSet.root points to the src/main/java or src/main/kotlin. All good here.
2. But fileSet.data.customContentRoot differ. For the test it's null but in IntelliJ, it points to src/main.

It seems like the test setup is not the same as in real IntelliJ.

Sorry, but it's quite impossible to understand and reproduce your issues without access to the full sources.

Yeah, I understand. The sources are here: https://github.com/lobodpav/spock-intellij-plugin

Replication steps:

1. Checkout the https://github.com/lobodpav/spock-intellij-plugin/tree/problem/test-setup-multiple-source-roots branch.
2. Run the io.github.lobodpav.spock.action.NewSpockSpecificationActionIdeaSpec#The action is available only when Groovy and Spock are on the classpath test.
3. For the first 3 items in the data table, the test fails. For instance, the isAvailable method returns true  for main/java (which is incorrect and the action is unavailable in real IntelliJ).