swarm/pss: fix data race on HandshakeController.symKeyIndex (#19162)

* swarm/pss: fix data race on HandshakeController.symKeyIndex

The HandshakeController.symKeyIndex map was accessed concurrently.
Since insufficient test coverage the race is not detected every time.
However, running TestClientHandshake a 100 times seems to be enough to
reproduce the race.

Note: I've chosen HandshakeController.lock to protect
HandshakeController.symKeyIndex as that was already protected in a few
functions by that lock.

Additionally:
- removed unused testStore
- enabled tests in handshake_test.go as they pass
- removed code duplication by adding getSymKey()

* swarm/pss: fix a data race on HandshakeController.keyC

* swarm/pss: fix data races with on Pss.symKeyPool

swarm/pss/client/client_test.go

@@ -23,7 +23,6 @@ import (
 	"fmt"
 	"math/rand"
 	"os"
-	"sync"
 	"testing"
 	"time"
 
@@ -286,18 +285,3 @@ func newServices() adapters.Services {
 		},
 	}
 }
-
-// copied from swarm/network/protocol_test_go
-type testStore struct {
-	sync.Mutex
-
-	values map[string][]byte
-}
-
-func (t *testStore) Load(key string) ([]byte, error) {
-	return nil, nil
-}
-
-func (t *testStore) Save(key string, v []byte) error {
-	return nil
-}