Prevent port re-use in HTTP API tests (#4745)

## Issue Addressed

CI is plagued by `AddrAlreadyInUse` failures, which are caused by race conditions in allocating free ports.

This PR removes all usages of the `unused_port` crate for Lighthouse's HTTP API, in favour of passing `:0` as the listen address. As a result, the listen address isn't known ahead of time and must be read from the listening socket after it binds. This requires tying some self-referential knots, which is a little disruptive, but hopefully doesn't clash too much with Deneb 🤞

There are still a few usages of `unused_tcp4_port` left in cases where we start external processes, like the `watch` Postgres DB, Anvil, Geth, Nethermind, etc. Removing these usages is non-trivial because it's hard to read the port back from an external process after starting it with `--port 0`. We might be able to do something on Linux where we read from `/proc/`, but I'll leave that for future work.

beacon_node/execution_layer/src/test_utils/mod.rs

@@ -25,7 +25,7 @@ use warp::{http::StatusCode, Filter, Rejection};
 use crate::EngineCapabilities;
 pub use execution_block_generator::{generate_pow_block, Block, ExecutionBlockGenerator};
 pub use hook::Hook;
-pub use mock_builder::{Context as MockBuilderContext, MockBuilder, Operation, TestingBuilder};
+pub use mock_builder::{Context as MockBuilderContext, MockBuilder, MockBuilderServer, Operation};
 pub use mock_execution_layer::MockExecutionLayer;
 
 pub const DEFAULT_TERMINAL_DIFFICULTY: u64 = 6400;