[#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/exists_test.go

@@ -25,7 +25,7 @@ func BenchmarkExists(b *testing.B) {
 }
 
 func benchmarkExists(b *testing.B, shardNum int) {
-	e := testNewEngine(b).setShardsNum(b, shardNum).engine
+	e := testNewEngine(b).setShardsNum(b, shardNum).prepare(b).engine
 	defer func() { require.NoError(b, e.Close(context.Background())) }()
 
 	addr := oidtest.Address()