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/execution_layer/src/lib.rs

@@ -304,11 +304,7 @@ impl ExecutionLayer {
         T: Fn(&'a Self) -> U,
         U: Future<Output = Result<V, Error>>,
     {
-        let runtime = self
-            .executor()
-            .runtime()
-            .upgrade()
-            .ok_or(Error::ShuttingDown)?;
+        let runtime = self.executor().handle().ok_or(Error::ShuttingDown)?;
         // TODO(merge): respect the shutdown signal.
         runtime.block_on(generate_future(self))
     }
@@ -322,11 +318,7 @@ impl ExecutionLayer {
         T: Fn(&'a Self) -> U,
         U: Future<Output = V>,
     {
-        let runtime = self
-            .executor()
-            .runtime()
-            .upgrade()
-            .ok_or(Error::ShuttingDown)?;
+        let runtime = self.executor().handle().ok_or(Error::ShuttingDown)?;
         // TODO(merge): respect the shutdown signal.
         Ok(runtime.block_on(generate_future(self)))
     }
@@ -1263,13 +1255,15 @@ impl ExecutionLayer {
 mod test {
     use super::*;
     use crate::test_utils::MockExecutionLayer as GenericMockExecutionLayer;
+    use task_executor::test_utils::TestRuntime;
     use types::MainnetEthSpec;
 
     type MockExecutionLayer = GenericMockExecutionLayer<MainnetEthSpec>;
 
     #[tokio::test]
     async fn produce_three_valid_pos_execution_blocks() {
-        MockExecutionLayer::default_params()
+        let runtime = TestRuntime::default();
+        MockExecutionLayer::default_params(runtime.task_executor.clone())
             .move_to_terminal_block()
             .produce_valid_execution_payload_on_head()
             .await
@@ -1281,7 +1275,8 @@ mod test {
 
     #[tokio::test]
     async fn finds_valid_terminal_block_hash() {
-        MockExecutionLayer::default_params()
+        let runtime = TestRuntime::default();
+        MockExecutionLayer::default_params(runtime.task_executor.clone())
             .move_to_block_prior_to_terminal_block()
             .with_terminal_block(|spec, el, _| async move {
                 el.engines().upcheck_not_synced(Logging::Disabled).await;
@@ -1300,7 +1295,8 @@ mod test {
 
     #[tokio::test]
     async fn verifies_valid_terminal_block_hash() {
-        MockExecutionLayer::default_params()
+        let runtime = TestRuntime::default();
+        MockExecutionLayer::default_params(runtime.task_executor.clone())
             .move_to_terminal_block()
             .with_terminal_block(|spec, el, terminal_block| async move {
                 el.engines().upcheck_not_synced(Logging::Disabled).await;
@@ -1316,7 +1312,8 @@ mod test {
 
     #[tokio::test]
     async fn rejects_invalid_terminal_block_hash() {
-        MockExecutionLayer::default_params()
+        let runtime = TestRuntime::default();
+        MockExecutionLayer::default_params(runtime.task_executor.clone())
             .move_to_terminal_block()
             .with_terminal_block(|spec, el, terminal_block| async move {
                 el.engines().upcheck_not_synced(Logging::Disabled).await;
@@ -1334,7 +1331,8 @@ mod test {
 
     #[tokio::test]
     async fn rejects_unknown_terminal_block_hash() {
-        MockExecutionLayer::default_params()
+        let runtime = TestRuntime::default();
+        MockExecutionLayer::default_params(runtime.task_executor.clone())
             .move_to_terminal_block()
             .with_terminal_block(|spec, el, _| async move {
                 el.engines().upcheck_not_synced(Logging::Disabled).await;