add sighup tests

[evgeniiz321]

there is a problem during fschain endpoint update:

info	neofs-node/config.go:902	SIGHUP has been received, rereading configuration...
info	shard/control.go:315	trying to restore read-write mode	{"shard_id": "EgZdmowYzd4UqrK63zvAFg"}
info	shard/mode.go:28	setting shard mode	{"shard_id": "EgZdmowYzd4UqrK63zvAFg", "old_mode": "READ_WRITE", "new_mode": "READ_WRITE"}
info	shard/mode.go:69	shard mode set successfully	{"shard_id": "EgZdmowYzd4UqrK63zvAFg", "mode": "READ_WRITE"}
info	shard/control.go:315	trying to restore read-write mode	{"shard_id": "WQ1oGrsfXW2xe54Uy5jcKS"}
info	shard/mode.go:28	setting shard mode	{"shard_id": "WQ1oGrsfXW2xe54Uy5jcKS", "old_mode": "READ_WRITE", "new_mode": "READ_WRITE"}
info	shard/mode.go:69	shard mode set successfully	{"shard_id": "WQ1oGrsfXW2xe54Uy5jcKS", "mode": "READ_WRITE"}
info	client/notifications.go:228	RPC connection lost, attempting reconnect
info	neofs-node/config.go:960	configuration has been reloaded successfully
info	client/multi.go:53	connection to RPC node has been established	{"endpoint": "ws://localhost:58302/ws"}
info	neofs-node/main.go:207	restarting internal services because of RPC connection loss...
info	event/listener.go:221	stop event listener by context	{"reason": "context canceled"}
warn	neofs-node/main.go:181	internal application error	{"message": "restarting after morph connection was lost: getting network state: could not get current epoch number: could not perform test invocation (epoch): chain/client: contract execution finished with state FAULT; exception: at instruction 31 (SYSCALL): System.Contract.Call failed: called contract 4ea20d322984f445a4a737dbbd4bb1900cdee905 not found: key not found"}
info	neofs-node/grpc.go:138	stopping gRPC server...	{"name": "NeoFS Control API"}
error	client/notifications.go:267	could not restore block subscription	{"error": "connection lost: while waiting for the response"}
info	client/notifications.go:228	RPC connection lost, attempting reconnect
info	neofs-node/grpc.go:154	gRPC server stopped successfully	{"name": "NeoFS Control API"}
info	neofs-node/storage.go:65	closing components of the storage engine...
warn	client/multi.go:43	could not establish connection to RPC node	{"endpoint": "ws://localhost:58302/ws", "error": "WS client creation: dial tcp: lookup localhost: operation was canceled"}
debug	blobstor/control.go:44	closing...	{"shard_id": "EgZdmowYzd4UqrK63zvAFg"}
info	shard/gc.go:165	waiting for GC workers to stop...
debug	shard/gc.go:151	GC is stopped
warn	shard/gc.go:105	stop event listener by closed channel
debug	blobstor/control.go:44	closing...	{"shard_id": "WQ1oGrsfXW2xe54Uy5jcKS"}
info	shard/gc.go:165	waiting for GC workers to stop...
debug	shard/gc.go:151	GC is stopped
warn	shard/gc.go:105	stop event listener by closed channel
info	neofs-node/storage.go:73	all components of the storage engine closed successfully
info	neofs-node/grpc.go:138	stopping gRPC server...	{"name": "NeoFS Public API"}
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x20 pc=0x1009fed98]

goroutine 70 [running]:
github.com/nspcc-dev/neofs-node/pkg/morph/client.(*Client).routeNotifications(0x140001be240)
	github.com/nspcc-dev/neofs-node/pkg/morph/client/notifications.go:236 +0xf8
created by github.com/nspcc-dev/neofs-node/pkg/morph/client.New in goroutine 1
	github.com/nspcc-dev/neofs-node/pkg/morph/client/constructor.go:172 +0x630
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x1009ff044]

goroutine 372 [running]:
github.com/nspcc-dev/neofs-node/pkg/morph/client.(*Client).restoreSubscriptions(0x140001be240, 0x0, 0x140001172c0)
	github.com/nspcc-dev/neofs-node/pkg/morph/client/notifications.go:265 +0xc4
created by github.com/nspcc-dev/neofs-node/pkg/morph/client.(*Client).routeNotifications in goroutine 70
	github.com/nspcc-dev/neofs-node/pkg/morph/client/notifications.go:230 +0xe4

Besides panic there is a -

warn	neofs-node/main.go:181	internal application error	{"message": "restarting after morph connection was lost: getting network state: could not get current epoch number: could not perform test invocation (epoch): chain/client: contract execution finished with state FAULT; exception: at instruction 31 (SYSCALL): System.Contract.Call failed: called contract 4ea20d322984f445a4a737dbbd4bb1900cdee905 not found: key not found"}

I need some help to understand why it happens

[roman-khimov]

Node can't panic, if it does --- it's a bug. RPC error looks as if you're trying to access some different network. Contract hash is cached, it can not change between reconnects to the same network, but in your case calls to this contract fail because the contract doesn't exist.