backend: move backend implementation helpers to util package

This removes code that is only used within a backend implementation from
the backend package. The latter now only contains code that also has
external users.

internal/backend/util/foreground_test.go

@@ -0,0 +1,41 @@
+//go:build !windows
+// +build !windows
+
+package util_test
+
+import (
+	"bufio"
+	"os"
+	"os/exec"
+	"strings"
+	"testing"
+
+	"github.com/restic/restic/internal/backend/util"
+	rtest "github.com/restic/restic/internal/test"
+)
+
+func TestForeground(t *testing.T) {
+	err := os.Setenv("RESTIC_PASSWORD", "supersecret")
+	rtest.OK(t, err)
+
+	cmd := exec.Command("env")
+	stdout, err := cmd.StdoutPipe()
+	rtest.OK(t, err)
+
+	bg, err := util.StartForeground(cmd)
+	rtest.OK(t, err)
+	defer func() {
+		rtest.OK(t, cmd.Wait())
+	}()
+
+	err = bg()
+	rtest.OK(t, err)
+
+	sc := bufio.NewScanner(stdout)
+	for sc.Scan() {
+		if strings.HasPrefix(sc.Text(), "RESTIC_PASSWORD=") {
+			t.Error("subprocess got to see the password")
+		}
+	}
+	rtest.OK(t, err)
+}