testlib: add tests for Map.entrySet().stream() and Spliterator charac…

[faisalmastoi55]

Adds tests for 'Map.entrySet().stream()' and its associated 'Spliterator' characteristics as requested in issue #7855.

Validates that:

• 'map.entrySet().stream().toList()' returns exactly the same entries as the original map.
• For maps that allow null values (e.g., HashMap), the 'Spliterator.NONNULL' characteristic is not present if nulls exist.
• For maps that disallow nulls (e.g., ImmutableMap), the 'Spliterator.NONNULL' characteristic is present.

Test coverage includes:

• Basic map entries streaming validation.
• Validation for Spliterator characteristics with and without nulls.

These tests help ensure compliance with Java 8+ stream semantics and provide regression protection for future implementations.

Fixes #7855.

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