fix wal close not waiting for threads, better test harness

Signed-off-by: Alex Chi <iskyzh@gmail.com>

mini-lsm-mvcc/src/lsm_storage.rs

@@ -188,12 +188,6 @@ impl MiniLsm {
         self.compaction_notifier.send(()).ok();
         self.flush_notifier.send(()).ok();
 
-        if self.inner.options.enable_wal {
-            self.inner.sync()?;
-            self.inner.sync_dir()?;
-            return Ok(());
-        }
-
         let mut compaction_thread = self.compaction_thread.lock();
         if let Some(compaction_thread) = compaction_thread.take() {
             compaction_thread
@@ -207,6 +201,12 @@ impl MiniLsm {
                 .map_err(|e| anyhow::anyhow!("{:?}", e))?;
         }
 
+        if self.inner.options.enable_wal {
+            self.inner.sync()?;
+            self.inner.sync_dir()?;
+            return Ok(());
+        }
+
         // create memtable and skip updating manifest
         if !self.inner.state.read().memtable.is_empty() {
             self.inner