[#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/evacuate_test.go

@@ -75,10 +75,9 @@ func newEngineEvacuate(t *testing.T, shardNum int, objPerShard int) (*StorageEng
 					pilorama.WithPerm(0o700),
 				),
 			}
-		})
+		}).
+		prepare(t)
 	e, ids := te.engine, te.shardIDs
-	require.NoError(t, e.Open(context.Background()))
-	require.NoError(t, e.Init(context.Background()))
 
 	objects := make([]*objectSDK.Object, 0, objPerShard*len(ids))
 	treeID := "version"