[#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/lock_test.go

@@ -57,11 +57,9 @@ func TestLockUserScenario(t *testing.T) {
 				}),
 				shard.WithTombstoneSource(tss{lockerExpiresAfter}),
 			}
-		})
+		}).
+		prepare(t)
 	e := testEngine.engine
-	require.NoError(t, e.Open(context.Background()))
-	require.NoError(t, e.Init(context.Background()))
-
 	defer func() { require.NoError(t, e.Close(context.Background())) }()
 
 	lockerID := oidtest.ID()
@@ -162,11 +160,9 @@ func TestLockExpiration(t *testing.T) {
 					return pool
 				}),
 			}
-		})
+		}).
+		prepare(t)
 	e := testEngine.engine
-	require.NoError(t, e.Open(context.Background()))
-	require.NoError(t, e.Init(context.Background()))
-
 	defer func() { require.NoError(t, e.Close(context.Background())) }()
 
 	const lockerExpiresAfter = 13
@@ -243,9 +239,8 @@ func TestLockForceRemoval(t *testing.T) {
 				}),
 				shard.WithDeletedLockCallback(e.processDeletedLocks),
 			}
-		}).engine
-	require.NoError(t, e.Open(context.Background()))
-	require.NoError(t, e.Init(context.Background()))
+		}).
+		prepare(t).engine
 	defer func() { require.NoError(t, e.Close(context.Background())) }()
 
 	cnr := cidtest.ID()