[#1491] engine/test: Rework engine test utils

- Remove `testNewShard` and `setInitializedShards` because they
violated the default engine workflow. The correct workflow is:
first use `New()`, followed by `Open()`, and then `Init()`. As a
result, adding new logic to `(*StorageEngine).Init` caused several
tests to fail with a panic when attempting to access uninitialized
resources. Now, all engines created with the test utils must be
initialized manually. The new helper method `prepare` can be used
for that purpose.
- Additionally, `setInitializedShards` hardcoded the shard worker
pool size, which prevented it from being configured in tests and
benchmarks. This has been fixed as well.
- Ensure engine initialization is done wherever it was missing.
- Refactor `setShardsNumOpts`, `setShardsNumAdditionalOpts`, and
`setShardsNum`. Make them all depend on `setShardsNumOpts`.

Signed-off-by: Aleksey Savchuk <a.savchuk@yadro.com>

pkg/local_object_storage/engine/control_test.go

@@ -164,7 +164,7 @@ func testEngineFailInitAndReload(t *testing.T, degradedMode bool, opts []shard.O
 }
 
 func TestExecBlocks(t *testing.T) {
-	e := testNewEngine(t).setShardsNum(t, 2).engine // number doesn't matter in this test, 2 is several but not many
+	e := testNewEngine(t).setShardsNum(t, 2).prepare(t).engine // number doesn't matter in this test, 2 is several but not many
 
 	// put some object
 	obj := testutil.GenerateObjectWithCID(cidtest.ID())
@@ -302,7 +302,8 @@ func engineWithShards(t *testing.T, path string, num int) (*StorageEngine, []str
 					meta.WithEpochState(epochState{}),
 				),
 			}
-		})
+		}).
+		prepare(t)
 	e, ids := te.engine, te.shardIDs
 
 	for _, id := range ids {
@@ -312,8 +313,5 @@ func engineWithShards(t *testing.T, path string, num int) (*StorageEngine, []str
 	require.Equal(t, num, len(e.shards))
 	require.Equal(t, num, len(e.shardPools))
 
-	require.NoError(t, e.Open(context.Background()))
-	require.NoError(t, e.Init(context.Background()))
-
 	return e, currShards
 }