Add tests for extractor

[IITI-tushar]

This pull request adds comprehensive unit tests for the ExtractorChain functionality in the pkg/attestation/extractor_test.go file. The new tests verify the registration and retrieval of extractors, supported types, registration of custom extractors, and the behavior of extracting files from attestations—including deduplication and filtering.

Key additions and improvements:

ExtractorChain registration and retrieval:

• Adds tests to ensure that default extractors (material, command-run, product) are registered and retrievable, and that unknown types are not registered.
• Adds a test to verify that custom extractors can be registered and retrieved as expected.

Supported types:

• Adds a test to confirm that SupportedTypes() returns the expected set of extractor types.

File extraction behavior:

• Adds tests for ExtractAll() to check deduplication of file paths, correct application of type filters, and correct invocation count of each extractor.