Generalize sync block lookup tests

[dapplion]

Issue Addressed

As noted in

• https://github.com/sigp/lighthouse/pull/6497#discussion_r1803877443

The existing range sync tests should adopt the event-driven style of block lookup tests.

Proposed Changes

This PR just moves the existing tests to a new directory structure so git doesn't trip and retains all the sweet blame in the 3000 lines of code of lookup tests. A future PR will migrate the range sync tests.

beacon_node/network/src/sync/tests/
├── lookups.rs
├── mod.rs
└── range.rs

The idea is that lookup and range will each impl TestRig for what they need to keep code separate. The common functions can either be made pub(crate) (if already in lookups.rs) or moved to mod.rs.