fix: fix async sharding tests (#49)

`open - empty` was failing with an uncaught exception but this wasn't failing mocha or karma.

Found this problem testing playwright-test in this repo.

src/sharding.js

@@ -1,6 +1,6 @@
 'use strict'
 
-const { Adapter, Key, utils: { utf8Encoder } } = require('interface-datastore')
+const { Adapter, Key, utils: { utf8Encoder }, Errors } = require('interface-datastore')
 const sh = require('./shard')
 const KeytransformStore = require('./keytransform')
 
@@ -100,6 +100,9 @@ class ShardingDatastore extends Adapter {
    */
   static async create (store, shard) {
     const hasShard = await store.has(shardKey)
+    if (!hasShard && !shard) {
+      throw Errors.dbOpenFailedError(Error('Shard is required when datastore doesn\'t have a shard key already.'))
+    }
     if (!hasShard) {
       // @ts-ignore i have no idea what putRaw is or saw any implementation
       const put = typeof store.putRaw === 'function' ? store.putRaw.bind(store) : store.put.bind(store)

test/sharding.spec.js

@@ -21,21 +21,21 @@ describe('ShardingStore', () => {
     expect(utf8Decoder.decode(res[1])).to.eql(sh.readme)
   })
 
-  it('open - empty', async () => {
+  it('open - empty', () => {
     const ms = new MemoryDatastore()
     // @ts-expect-error
     const store = new ShardingStore(ms)
-    expect(store.open())
+    return expect(store.open())
       .to.eventually.be.rejected()
-      .with.property('code', 'ERR_NOT_FOUND')
+      .with.property('code', 'ERR_DB_OPEN_FAILED')
   })
 
-  it('open - existing', async () => {
+  it('open - existing', () => {
     const ms = new MemoryDatastore()
     const shard = new sh.NextToLast(2)
     const store = new ShardingStore(ms, shard)
 
-    expect(store.open()).to.eventually.be.fulfilled()
+    return expect(store.open()).to.eventually.be.fulfilled()
   })
 
   it('basics', async () => {