swarm: TestSwarmUpdate: remove non-existing "--quiet" flag

The `docker swarm update` copmmand does not have a `--quiet` flag, but this
test was trying to set it.

    docker swarm update --help

    Usage:  docker swarm update [OPTIONS]

    Update the swarm

    Options:
          --autolock                        Change manager autolocking setting (true|false)
          --cert-expiry duration            Validity period for node certificates (ns|us|ms|s|m|h) (default 2160h0m0s)
          --dispatcher-heartbeat duration   Dispatcher heartbeat period (ns|us|ms|s|m|h) (default 5s)
          --external-ca external-ca         Specifications of one or more certificate signing endpoints
          --max-snapshots uint              Number of additional Raft snapshots to retain
          --snapshot-interval uint          Number of log entries between Raft snapshots (default 10000)
          --task-history-limit int          Task history retention limit (default 5)

The test didn't catch this issue, because errors when setting the flag were
not handled, so also adding error-handling;

    === Failed
    === FAIL: cli/command/swarm TestSwarmUpdate (0.00s)
        update_test.go:177: assertion failed: error is not nil: no such flag -quiet

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/swarm/update_test.go

@@ -105,7 +105,6 @@ func TestSwarmUpdate(t *testing.T) {
 				flagMaxSnapshots:        "10",
 				flagSnapshotInterval:    "100",
 				flagAutolock:            "true",
-				flagQuiet:               "true",
 			},
 			swarmInspectFunc: func() (swarm.Swarm, error) {
 				return *swarmInfo, nil
@@ -174,7 +173,7 @@ func TestSwarmUpdate(t *testing.T) {
 		cmd := newUpdateCommand(cli)
 		cmd.SetArgs(tc.args)
 		for key, value := range tc.flags {
-			cmd.Flags().Set(key, value)
+			assert.Check(t, cmd.Flags().Set(key, value))
 		}
 		cmd.SetOut(cli.OutBuffer())
 		assert.NilError(t, cmd.Execute())