Allow TaskExecutor to be used in async tests (#3178)

# Description

Since the `TaskExecutor` currently requires a `Weak<Runtime>`, it's impossible to use it in an async test where the `Runtime` is created outside our scope. Whilst we *could* create a new `Runtime` instance inside the async test, dropping that `Runtime` would cause a panic (you can't drop a `Runtime` in an async context).

To address this issue, this PR creates the `enum Handle`, which supports either:

- A `Weak<Runtime>` (for use in our production code)
- A `Handle` to a runtime (for use in testing)

In theory, there should be no change to the behaviour of our production code (beyond some slightly different descriptions in HTTP 500 errors), or even our tests. If there is no change, you might ask *"why bother?"*. There are two PRs (#3070 and #3175) that are waiting on these fixes to introduce some new tests. Since we've added the EL to the `BeaconChain` (for the merge), we are now doing more async stuff in tests.

I've also added a `RuntimeExecutor` to the `BeaconChainTestHarness`. Whilst that's not immediately useful, it will become useful in the near future with all the new async testing.

beacon_node/beacon_chain/src/builder.rs

@@ -27,7 +27,7 @@ use std::marker::PhantomData;
 use std::sync::Arc;
 use std::time::Duration;
 use store::{Error as StoreError, HotColdDB, ItemStore, KeyValueStoreOp};
-use task_executor::ShutdownReason;
+use task_executor::{ShutdownReason, TaskExecutor};
 use types::{
     BeaconBlock, BeaconState, ChainSpec, Checkpoint, EthSpec, Graffiti, Hash256, PublicKeyBytes,
     Signature, SignedBeaconBlock, Slot,
@@ -91,6 +91,7 @@ pub struct BeaconChainBuilder<T: BeaconChainTypes> {
     // Pending I/O batch that is constructed during building and should be executed atomically
     // alongside `PersistedBeaconChain` storage when `BeaconChainBuilder::build` is called.
     pending_io_batch: Vec<KeyValueStoreOp>,
+    task_executor: Option<TaskExecutor>,
 }
 
 impl<TSlotClock, TEth1Backend, TEthSpec, THotStore, TColdStore>
@@ -129,6 +130,7 @@ where
             slasher: None,
             validator_monitor: None,
             pending_io_batch: vec![],
+            task_executor: None,
         }
     }
 
@@ -182,6 +184,13 @@ where
         self.log = Some(log);
         self
     }
+
+    /// Sets the task executor.
+    pub fn task_executor(mut self, task_executor: TaskExecutor) -> Self {
+        self.task_executor = Some(task_executor);
+        self
+    }
+
     /// Attempt to load an existing eth1 cache from the builder's `Store`.
     pub fn get_persisted_eth1_backend(&self) -> Result<Option<SszEth1>, String> {
         let store = self
@@ -919,6 +928,7 @@ mod test {
     use std::time::Duration;
     use store::config::StoreConfig;
     use store::{HotColdDB, MemoryStore};
+    use task_executor::test_utils::TestRuntime;
     use types::{EthSpec, MinimalEthSpec, Slot};
 
     type TestEthSpec = MinimalEthSpec;
@@ -952,10 +962,12 @@ mod test {
         .expect("should create interop genesis state");
 
         let (shutdown_tx, _) = futures::channel::mpsc::channel(1);
+        let runtime = TestRuntime::default();
 
         let chain = BeaconChainBuilder::new(MinimalEthSpec)
             .logger(log.clone())
             .store(Arc::new(store))
+            .task_executor(runtime.task_executor.clone())
             .genesis_state(genesis_state)
             .expect("should build state using recent genesis")
             .dummy_eth1_backend()