Make garbage collection test less failure prone (#3599)

## Issue Addressed

NA

## Proposed Changes

This PR attempts to fix the following spurious CI failure:

```
---- store_tests::garbage_collect_temp_states_from_failed_block stdout ----
thread 'store_tests::garbage_collect_temp_states_from_failed_block' panicked at 'disk store should initialize: DBError { message: "Error { message: \"IO error: lock /tmp/.tmp6DcBQ9/cold_db/LOCK: already held by process\" }" }', beacon_node/beacon_chain/tests/store_tests.rs:59:10
```

I believe that some async task is taking a clone of the store and holding it in some other thread for a short time. This creates a race-condition when we try to open a new instance of the store.

## Additional Info

NA

beacon_node/beacon_chain/tests/store_tests.rs

@@ -26,6 +26,7 @@ use store::{
     HotColdDB, LevelDB, StoreConfig,
 };
 use tempfile::{tempdir, TempDir};
+use tokio::time::sleep;
 use tree_hash::TreeHash;
 use types::test_utils::{SeedableRng, XorShiftRng};
 use types::*;
@@ -1985,16 +1986,16 @@ async fn pruning_test(
     check_no_blocks_exist(&harness, stray_blocks.values());
 }
 
-#[test]
-fn garbage_collect_temp_states_from_failed_block() {
+#[tokio::test]
+async fn garbage_collect_temp_states_from_failed_block() {
     let db_path = tempdir().unwrap();
+
+    // Wrap these functions to ensure the variables are dropped before we try to open another
+    // instance of the store.
+    let mut store = {
         let store = get_store(&db_path);
         let harness = get_harness(store.clone(), LOW_VALIDATOR_COUNT);
 
-    // Use a `block_on_dangerous` rather than an async test to stop spawned processes from holding
-    // a reference to the store.
-    harness.chain.task_executor.clone().block_on_dangerous(
-        async move {
         let slots_per_epoch = E::slots_per_epoch();
 
         let genesis_state = harness.get_current_state();
@@ -2021,9 +2022,19 @@ fn garbage_collect_temp_states_from_failed_block() {
             store.iter_temporary_state_roots().count(),
             block_slot.as_usize() - 1
         );
-        },
-        "test",
-    );
+        store
+    };
+
+    // Wait until all the references to the store have been dropped, this helps ensure we can
+    // re-open the store later.
+    loop {
+        store = if let Err(store_arc) = Arc::try_unwrap(store) {
+            sleep(Duration::from_millis(500)).await;
+            store_arc
+        } else {
+            break;
+        }
+    }
 
     // On startup, the store should garbage collect all the temporary states.
     let store = get_store(&db_path);