blockstore-test: use try/catch instead of promise.catch/finally.

finally was introduce in node v10, so this test will fail on v8.6.0 node.

test/blockstore-test.js

@@ -813,17 +813,20 @@ describe('BlockStore', function() {
         // Accidentally don't use `await` and attempt to
         // write multiple blocks in parallel and at the
         // same file position.
-        const promise = store.write(hash, block);
+        (async () => {
-        promise.catch((e) => {
+          try {
+            await store.write(hash, block);
+          } catch (e) {
             err = e;
-        }).finally(() => {
+          } finally {
             finished += 1;
             if (finished >= 16) {
               assert(err);
               assert(err.message, 'Already writing.');
               done();
             }
-        });
+          }
+        })();
       }
     });