SystemPropertyHelper is not thread safe

[sleberknight]

SystemPropertyHelper is used by tests to verify behavior of ConfigProvider classes that support system properties.

This class uses a static Set<String and has only static methods, which means it is not thread-safe and therefore cannot ever be used by parallel tests.

Rather than trying to fix this ourselves, we should consider using JUnit Pioneer's ability to set and clear system properties. It attempts to handle thread-safety by using JUnit 5 resource locks to prevent tests annotated with its annotations won't execute in parallel. They also note that it cannot cover all possible cases, which is important to understand.