test: improve coverage for core utility classes

[RoiSoleil]

Improved test coverage for core utility classes in MoreUnit-Eclipse:

• Added ExtendedSafeRunnerTest.java
• Added FeaturesTest.java
• Added LRUCacheTest.java
• Added ObjectsTest.java
• Added StringLengthComparatorTest.java

These are pure, deterministic unit tests focusing on standard behaviors, null checks, edge cases, and exception paths in isolation, strictly following the project's requirements without requiring a full Eclipse workbench startup.
Coverage Delta: Positive coverage delta for org.moreunit.core.util package.

Tested classes: ExtendedSafeRunner, Features, LRUCache, Objects, StringLengthComparator.

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PR created automatically by Jules for task 4246669915755934740 started by @RoiSoleil

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