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/beacon_chain/tests/store_tests.rs

@@ -31,7 +31,9 @@ use fork_choice::PayloadStatus;
 use logging::create_test_tracing_subscriber;
 use maplit::hashset;
 use rand::Rng;
+use rand::SeedableRng;
 use rand::rngs::StdRng;
+use rand_xorshift::XorShiftRng;
 use slot_clock::{SlotClock, TestingSlotClock};
 use ssz_types::VariableList;
 use state_processing::{BlockReplayer, state_advance::complete_state_advance};
@@ -50,7 +52,6 @@ use store::{
 };
 use tempfile::{TempDir, tempdir};
 use tracing::info;
-use types::test_utils::{SeedableRng, XorShiftRng};
 use types::*;
 
 // Should ideally be divisible by 3.