How do you organize tests in a modular Java project?

I am creating a modular build (using module-info.java) on GitHub, but when adding a module-info.java to the modules that I want modular, no tests can be executed...

How can I achieve this?

I am using the following versions:

junit.jupiter version 5.3.0 (first take was also unsuccessful with version 5.2.0)
maven-compiler-plugin version 3.8.0 (first take was also unsuccessful with version 3.7.0)
maven-surefire-plugin version 2.22.0 (first take was also unsuccessful with version 2.21.0)

A typical error from the failing tests looks like:

java.lang.reflect.InaccessibleObjectException: Unable to make com.github.jactor.rises.commons.dto.UserDtoTest() accessible: module jactor.rises.commons does not "opens com.github.jactor.rises.commons.dto" to unnamed module @65e98b1c

Welcome to Testing In The Modular World!

Which kind of tests do you want write?

Black-box tests: Create a test-only project (no "src/main" directory) and declare a "src/test/java/module-info.java" module descriptor.

White-box tests: As it was from Day 1 you need to "blend in"/merge/shadow your test classes into your main classes or vice versa. Here you have mainly two ways to achieve this:

"compile modular main sources" and "patch plain test sources" at test-runtime with some additional "JVM options hacking the Module system" to execute tests.
"compile modular test sources" and "patch modular main sources" at compile-time to execute tests.