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/http_api/src/lib.rs

@@ -3635,12 +3635,13 @@ pub fn serve<T: BeaconChainTypes>(
                         // send the response back to our original HTTP request
                         // task via a channel.
                         let builder_future = async move {
-                            let builder = chain
+                            let arc_builder = chain
                                 .execution_layer
                                 .as_ref()
                                 .ok_or(BeaconChainError::ExecutionLayerMissing)
                                 .map_err(warp_utils::reject::beacon_chain_error)?
-                                .builder()
+                                .builder();
+                            let builder = arc_builder
                                 .as_ref()
                                 .ok_or(BeaconChainError::BuilderMissing)
                                 .map_err(warp_utils::reject::beacon_chain_error)?;

beacon_node/http_api/src/test_utils.rs

@@ -129,17 +129,9 @@ pub async fn create_api_server<T: BeaconChainTypes>(
     test_runtime: &TestRuntime,
     log: Logger,
 ) -> ApiServer<T::EthSpec, impl Future<Output = ()>> {
-    // Get a random unused port.
-    let port = unused_port::unused_tcp4_port().unwrap();
-    create_api_server_on_port(chain, test_runtime, log, port).await
-}
+    // Use port 0 to allocate a new unused port.
+    let port = 0;
 
-pub async fn create_api_server_on_port<T: BeaconChainTypes>(
-    chain: Arc<BeaconChain<T>>,
-    test_runtime: &TestRuntime,
-    log: Logger,
-    port: u16,
-) -> ApiServer<T::EthSpec, impl Future<Output = ()>> {
     let (network_senders, network_receivers) = NetworkSenders::new();
 
     // Default metadata