Activity indicator for init command (#207)

* first version of spinner

- implemented class with basic method
- covered with dummy test
- integrated with init command

* Style and license remarks

* fixed review remarks

* fixed typo + integration of spinner with terminate command

* integration of spinner with create command

cli/internal/cmd/spinner_test.go

@@ -0,0 +1,91 @@
+/*
+Copyright (c) Edgeless Systems GmbH
+
+SPDX-License-Identifier: AGPL-3.0-only
+*/
+
+package cmd
+
+import (
+	"bytes"
+	"fmt"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+const (
+	baseWait = 1
+	baseText = "Loading"
+)
+
+func TestSpinnerInitialState(t *testing.T) {
+	cmd := NewInitCmd()
+	var out bytes.Buffer
+	cmd.SetOut(&out)
+	var errOut bytes.Buffer
+	cmd.SetErr(&errOut)
+
+	s := newSpinner(cmd, baseText, true)
+	s.Start()
+	time.Sleep(baseWait * time.Second)
+	s.Stop()
+	require.True(t, out.Len() > 0)
+	require.True(t, errOut.Len() == 0)
+
+	outStr := out.String()
+	require.True(t, strings.HasPrefix(outStr, generateAllStatesAsString(baseText, true)))
+}
+
+func TestSpinnerFinalState(t *testing.T) {
+	cmd := NewInitCmd()
+	var out bytes.Buffer
+	cmd.SetOut(&out)
+	var errOut bytes.Buffer
+	cmd.SetErr(&errOut)
+
+	s := newSpinner(cmd, baseText, true)
+	s.Start()
+	time.Sleep(baseWait * time.Second)
+	s.Stop()
+	require.True(t, out.Len() > 0)
+	require.True(t, errOut.Len() == 0)
+
+	outStr := out.String()
+	require.True(t, strings.HasSuffix(outStr, baseText+"...  \n"))
+}
+
+func TestSpinnerDisabledShowDotsFlag(t *testing.T) {
+	cmd := NewInitCmd()
+	var out bytes.Buffer
+	cmd.SetOut(&out)
+	var errOut bytes.Buffer
+	cmd.SetErr(&errOut)
+
+	s := newSpinner(cmd, baseText, false)
+	s.Start()
+	time.Sleep(baseWait * time.Second)
+	s.Stop()
+	require.True(t, out.Len() > 0)
+	require.True(t, errOut.Len() == 0)
+
+	outStr := out.String()
+	require.True(t, strings.HasPrefix(outStr, generateAllStatesAsString(baseText, false)))
+	require.True(t, strings.HasSuffix(outStr, baseText+"  \n"))
+}
+
+func generateAllStatesAsString(text string, showDots bool) string {
+	var builder strings.Builder
+
+	for i := 0; i < len(spinnerStates); i++ {
+		dotsState := ""
+		if showDots {
+			dotsState = dotsStates[i%len(dotsStates)]
+		}
+		builder.WriteString(fmt.Sprintf("\r%s %s%s", spinnerStates[i], text, dotsState))
+	}
+
+	return builder.String()
+}