testlib: Add ConcurrentMapSpliteratorTester for spliterator characteristics

[nickita-khylkouski]

Summary

• Add ConcurrentMapSpliteratorTester to verify spliterator characteristics on ConcurrentMap views
• Tests verify that entrySet(), keySet(), and values() spliterators have the CONCURRENT characteristic
• Tests verify that no spliterator has both CONCURRENT and SIZED characteristics (mutually incompatible per Java spec)

Motivation

Per issue #7855 and maintainer guidance from @chaoren:

• ConcurrentMap implementations should return spliterators with the CONCURRENT characteristic
• The Java specification states that a top-level Spliterator should not report both CONCURRENT and SIZED

Currently, some Guava ConcurrentMap implementations (e.g., LocalCache, MapMakerInternalMap) return spliterators that:

• Do NOT have CONCURRENT (should have it)
• DO have SIZED (should not have it for concurrent collections)

This PR adds tests to document the expected behavior. A follow-up PR can fix the implementations.

Testing

• Tests follow the existing ConcurrentMap*Tester pattern in testlib
• Includes reflection helper methods for test suppression if needed
• Verified JDK's ConcurrentHashMap correctly returns CONCURRENT=true, SIZED=false

Related Issues

Fixes #7855