[#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/head_test.go

@@ -39,11 +39,11 @@ func TestHeadRaw(t *testing.T) {
 	link.SetSplitID(splitID)
 
 	t.Run("virtual object split in different shards", func(t *testing.T) {
-		s1 := testNewShard(t)
-		s2 := testNewShard(t)
+		te := testNewEngine(t).setShardsNum(t, 2).prepare(t)
+		e := te.engine
+		defer func() { require.NoError(t, e.Close(context.Background())) }()
 
-		e := testNewEngine(t).setInitializedShards(t, s1, s2).engine
-		defer e.Close(context.Background())
+		s1, s2 := te.shards[0], te.shards[1]
 
 		var putPrmLeft shard.PutPrm
 		putPrmLeft.SetObject(child)