System-Rules based tests migration

[remkop]

Related to #1930:

Problem

DefaultProviderTest and HelpAnsiTest tests using JUnit 4 and Stephan Birkner’s System-Rules are broken on Java 18-ea (UnsupportedOperationException for setSecurityManager).

I also saw some InaccessibleObjectException errors in the CI/CD build logs on Java 16+:

picocli.HelpAnsiTest > testTextWithMultipleStyledSections FAILED
    java.lang.reflect.InaccessibleObjectException at AccessibleObject.java:354

Analysis

The UnsupportedOperationException in Java 18-ea is due to the EnvironmentVariables rule reflection no longer working due to removal of the System::setSecurityManager API in Java 18-ea. However, it looks like this API was not actually removed in Java 18's GA release.

It is not clear what caused the InaccessibleObjectException.
All tests seem to pass on Zulu Java 18 (with a warning):

WARNING: A terminally deprecated method in java.lang.System has been called

> Task :test
WARNING: System::setSecurityManager has been called by
     org.junit.contrib.java.lang.system.ProvideSecurityManager 
    (file:/home/runner/.gradle/caches/modules-2/files-2.1/com.github.stefanbirkner/
    system-rules/1.19.0/d541c9a1cff0dda32e2436c74562e2e4aa6c[88]  
    (https://github.com/remkop/picocli/actions/runs/3993249999/jobs/6849880365#step:9:89)cd/system-rules-1.19.0.jar)
WARNING: Please consider reporting this to the maintainers of 
    org.junit.contrib.java.lang.system.ProvideSecurityManager
WARNING: System::setSecurityManager will be removed in a future release

At this moment we are unable to test with Java 19 because Gradle 7.4 does not support it.
This problem will resurface when the System::setSecurityManager API is actually removed.

Solution

The solution is to move all tests that use the System-Rules EnvironmentVariables rule to the picocli-legacy-tests module.

Also, add a module picocli-tests-junit5 that uses JUnit 5 and System Lambda instead. Migrate Environment and System exit tests to this module.

This module can also host other tests that require Java 8 that are currently in the picocli-annotation-processing-tests module:

• https://github.com/remkop/picocli/blob/main/picocli-annotation-processing-tests/src/test/java/picocli/Issue1380Test.java
• https://github.com/remkop/picocli/blob/main/picocli-annotation-processing-tests/src/test/java/picocli/Issue1420Test.java

(Maybe rename picocli-annotation-processing-tests to picocli-tests-annotation-processing to align the name of all test modules? Also rename picocli-legacy-tests to picocli-tests-legacy.)