Remove TestRandom (#9006)

We  have a legacy `TestRandom` trait which generates random types for testing and fuzzing.
This function overlaps with `arbitrary` which is used very commonly in the ecosystem.


  Remove `TestRandom` and generate random type instances using `Arbitrary`.


Co-Authored-By: Mac L <mjladson@pm.me>

Co-Authored-By: Michael Sproul <michael@sigmaprime.io>

beacon_node/builder_client/Cargo.toml

@@ -16,5 +16,7 @@ serde = { workspace = true }
 serde_json = { workspace = true }
 
 [dev-dependencies]
+arbitrary = { workspace = true }
 mockito = { workspace = true }
 tokio = { workspace = true }
+types = { workspace = true, features = ["arbitrary"] }