testutil: support empty string cmd args

FakeCommand allows unit tests to check which command-line arguments
were passed, by recording a log of calls with their arguments.

The following examples do not work:

1. shutdown -r "+0" ""

2. echo ""

The last argument supplies the wall message, and an empty argument
is valid. Having this support in place allows code to simply pass
an empty argument for exec.Command(...).

The issue is that both the argument and command delimiter uses null
characters, which makes the command split operation split on the
first occurrence of two null chars, which breaks the examples above.

To add support for this:

- Change the command delimiter from \000\000 => \000\f\n\r

- Update the Calls() function to split the lines correctly.

internals/testutil/exec.go

@@ -92,17 +92,18 @@ type FakeCmd struct {
 // faking commands that need "\n" in their args (like zenity)
 // we use the following convention:
 // - generate \0 to separate args
-// - generate \0\0 to separate commands
+// - generate \0\f\n\r magic sequence to separate commands
 var scriptTpl = `#!/bin/bash
 printf "%%s" "$(basename "$0")" >> %[1]q
+
 printf '\0' >> %[1]q
 
 for arg in "$@"; do
      printf "%%s" "$arg" >> %[1]q
      printf '\0'  >> %[1]q
 done
 
-printf '\0' >> %[1]q
+printf '\f\n\r' >> %[1]q
 %s
 `
 
@@ -176,12 +177,11 @@ func (cmd *FakeCmd) Calls() [][]string {
 	if err != nil {
 		panic(err)
 	}
-	logContent := strings.TrimSuffix(string(raw), "\000")
+	logContent := strings.TrimSuffix(string(raw), "\000\f\n\r")
 
 	allCalls := [][]string{}
-	calls := strings.Split(logContent, "\000\000")
+	calls := strings.Split(logContent, "\000\f\n\r")
 	for _, call := range calls {
-		call = strings.TrimSuffix(call, "\000")
 		allCalls = append(allCalls, strings.Split(call, "\000"))
 	}
 	return allCalls