Add JUnit Platform Launcher

[jonatan-ivanov]

According to the official documentation, org.junit.platform:junit-platform-launcher is needed (as a testRuntimeOnly dependency). When it is missing, it seems the build does not fail with older JUnit versions. I think this is because Gradle has one built-in and it falls-back to it when the dependency is not added.

But with Gradle 8.10 and JUnit 5.12.0, this causes the following error:

[...]
Caused by: org.junit.platform.commons.JUnitException: TestEngine with ID 'junit-jupiter' failed to discover tests	
[...]
Caused by: org.junit.platform.commons.JUnitException: OutputDirectoryProvider not available; probably due to unaligned versions of the junit-platform-engine and junit-platform-launcher jars on the classpath/module path.
[...]

See:

• build scan
• Bump org.junit:junit-bom from 5.11.4 to 5.12.0 #334
• CircleCI Build