graph/db: let test alias be UTF-8 compatible

Later on we will store the Alias as a Text field in our sql impls of the
graph db. For Postgres, this field then MUST be a valid UTF-8 string.
This is also the case in general for the alias according to [bolt
7](https://github.com/lightning/bolts/blob/e1fa25cf00446f3a6a6abbbc9a617cae5b75e39f/07-routing-gossip.md?plain=1#L313)

graph/db/graph_test.go

@@ -75,7 +75,7 @@ func createLightningNode(priv *btcec.PrivateKey) *models.LightningNode {
 		AuthSigBytes:         testSig.Serialize(),
 		LastUpdate:           time.Unix(updateTime, 0),
 		Color:                color.RGBA{1, 2, 3, 0},
-		Alias:                "kek" + string(pub[:]),
+		Alias:                "kek" + hex.EncodeToString(pub),
 		Features:             testFeatures,
 		Addresses:            testAddrs,
 	}

graph/notifications_test.go

@@ -89,7 +89,7 @@ func createTestNode(t *testing.T) *models.LightningNode {
 		LastUpdate:           time.Unix(updateTime, 0),
 		Addresses:            testAddrs,
 		Color:                color.RGBA{1, 2, 3, 0},
-		Alias:                "kek" + string(pub[:]),
+		Alias:                "kek" + hex.EncodeToString(pub),
 		AuthSigBytes:         testSig.Serialize(),
 		Features:             testFeatures,
 	}