Simplify gossip tests

[StephenButtolph]

Why this should be merged

1. There are test helpers that aren't used externally, we can move them to files that are only accessible for testing.
2. Prefixing everything with test isn't really needed if it is defined in a test file.
3. Some of these helpers weren't used anywhere. Like NoOpGossiper and FullSet.Gossip.
4. We can simplify the usage of the helpers by using more specific types rather than using structs with a single field.

How this works

Just cleanup, no functional changes.

How this was tested

CI

Need to be documented in RELEASES.md?

No.

[Copilot]

Pull request overview

This PR refactors the gossip test infrastructure by moving test helpers from production code into test files and simplifying their implementation. The changes remove unused test utilities and improve code organization by using more specific types instead of wrapper structs.

Key changes:

• Moved test helper types (testTx, testSet, testMarshaller, NoOpGossiper, TestGossiper, FullSet) from production files to test files
• Simplified test helper implementation by using type aliases and function types instead of struct wrappers
• Removed unused test helpers that had no external usage

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
network/p2p/gossip/test_gossip.go	Deleted file containing test helpers that are now defined in test files
network/p2p/gossip/handler.go	Updated interface assertion to use the Gossipable interface instead of concrete test type
network/p2p/gossip/gossip_test.go	Added test helper types (tx, marshaller, setDouble, gossiperFunc, fullSet) previously defined in production code
network/p2p/gossip/gossip.go	Removed test helper types (NoOpGossiper, TestGossiper, FullSet) and updated interface assertions
network/p2p/gossip/bloom_test.go	Updated test data to use simplified tx type alias

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[StephenButtolph]

hold off until repo merger is done