Write access is allowed inside write-action in command in test

I have some plugin code I'm developing that does a simple modification to a Java class, to add an annotation. I've previously written another small plugin to do similar modifications, which worked fine. For the first plugin I didn't write any tests, for the second one I have written a test by extending the LightCodeInsightFixtureTestCase class.

My plugin code looks like this:

    javaPsiFacade = JavaPsiFacade.getInstance(event.getProject());
    elementFactory = javaPsiFacade.getElementFactory();

...

    log("Looking for Java class of name: " + className);
    PsiClass psiClass = javaPsiFacade.findClass(className, GlobalSearchScope.projectScope(project));
    log("Found class: " + psiClass);


    PsiIdentifier psiIdentifier = psiClass.getNameIdentifier();
    log("identifier: " + psiIdentifier);

    CommandProcessor.getInstance().executeCommand(project, () -> {
         // find the class declaration and add the @Entity annotation
       psiClass.addBefore(elementFactory.createAnnotationFromText("@javax.persistence.Entity", null), psiIdentifier);

    }, "Plugin name", "PluginGroup");

However when running the test I get the dreaded:

ERROR: Assertion failed: Write access is allowed inside write-action only (see com.intellij.openapi.application.Application.runWriteAction())

I'm confused because I used the CommandProcessor in my previous plugin and it seemed to work fine. Have I got an issue with it here? Or is this something to do with the test setup that I need to do?

Thanks for any advice.