[#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/error_test.go

@@ -67,10 +67,8 @@ func newEngineWithErrorThreshold(t testing.TB, dir string, errThreshold uint32)
 					pilorama.WithPath(filepath.Join(dir, fmt.Sprintf("%d.pilorama", id))),
 					pilorama.WithPerm(0o700)),
 			}
-		})
+		}).prepare(t)
 	e := te.engine
-	require.NoError(t, e.Open(context.Background()))
-	require.NoError(t, e.Init(context.Background()))
 
 	for i, id := range te.shardIDs {
 		testShards[i].id = id