cmd/swarm: respect --loglevel in run_test helpers (#17739)

When CLI tests were spanning new nodes, the log level verbosity was
hard coded as 6. So the Swarm process was always polluting the test
output with TRACE level logs.

Now `go test -v ./cmd/swarm -loglevel 0` works as expected.

cmd/swarm/run_test.go

@@ -242,7 +242,7 @@ func existingTestNode(t *testing.T, dir string, bzzaccount string) *testNode {
 		"--bzzaccount", bzzaccount,
 		"--bzznetworkid", "321",
 		"--bzzport", httpPort,
-		"--verbosity", "3",
+		"--verbosity", fmt.Sprint(*loglevel),
 	)
 	node.Cmd.InputLine(testPassphrase)
 	defer func() {
@@ -318,7 +318,7 @@ func newTestNode(t *testing.T, dir string) *testNode {
 		"--bzzaccount", account.Address.String(),
 		"--bzznetworkid", "321",
 		"--bzzport", httpPort,
-		"--verbosity", "3",
+		"--verbosity", fmt.Sprint(*loglevel),
 	)
 	node.Cmd.InputLine(testPassphrase)
 	defer func() {