testutil: new package to test run osbuild run functionality

This commit adds a new testutil.MockCommand() helper that will
mock a command in $PATH to allow easier testing of e.g. the
`image-builder-cli build` comamnd that will invoke osbuild.

internal/testutil/testutil.go

@@ -0,0 +1,62 @@
+package testutil
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+type MockCmd struct {
+	binDir string
+	name   string
+}
+
+func MockCommand(t *testing.T, name string, script string) *MockCmd {
+	mockCmd := &MockCmd{
+		binDir: t.TempDir(),
+		name:   name,
+	}
+
+	fullScript := `#!/bin/bash -e
+for arg in "$@"; do
+   echo -e -n "$arg\x0" >> "$0".run
+done
+echo >> "$0".run
+` + script
+
+	t.Setenv("PATH", mockCmd.binDir+":"+os.Getenv("PATH"))
+	err := os.WriteFile(filepath.Join(mockCmd.binDir, name), []byte(fullScript), 0755)
+	require.NoError(t, err)
+
+	return mockCmd
+}
+
+func (mc *MockCmd) Path() string {
+	return filepath.Join(mc.binDir, mc.name)
+}
+
+func (mc *MockCmd) Restore() error {
+	return os.RemoveAll(mc.binDir)
+}
+
+func (mc *MockCmd) Calls() [][]string {
+	b, err := os.ReadFile(mc.Path() + ".run")
+	if os.IsNotExist(err) {
+		return nil
+	}
+	if err != nil {
+		panic(err)
+	}
+	var calls [][]string
+	for _, line := range strings.Split(string(b), "\n") {
+		if line == "" {
+			continue
+		}
+		call := strings.Split(line, "\000")
+		calls = append(calls, call[0:len(call)-1])
+	}
+	return calls
+}