test_helpers: better function comments for InitFS and Mount

It's confusing that you must pass "-extpass" for Mount but not
for InitFS. Note that in the comment.

tests/test_helpers/helpers.go

@@ -125,10 +125,11 @@ func ResetTmpDir(createDirIV bool) {
 	}
 }
 
-// InitFS calls "gocryptfs -init" on a new directory in TmpDir, passing
-// "extraArgs" in addition to useful defaults.
+// InitFS creates a new empty cipherdir and calls
 //
-// The returned cipherdir has NO trailing slash.
+//     gocryptfs -q -init -extpass "echo test" -scryptn=10 $extraArgs $cipherdir
+//
+// It returns cipherdir without a trailing slash.
 func InitFS(t *testing.T, extraArgs ...string) string {
 	dir, err := ioutil.TempDir(TmpDir, "")
 	if err != nil {
@@ -149,7 +150,7 @@ func InitFS(t *testing.T, extraArgs ...string) string {
 	err = cmd.Run()
 	if err != nil {
 		if t != nil {
-			t.Fatalf("InitFS with args %v failed: %v", args, err)
+			t.Fatalf("InitFS with args %q failed: %v", args, err)
 		} else {
 			log.Panic(err)
 		}

tests/test_helpers/mount_unmount.go

@@ -26,6 +26,9 @@ type mountInfo struct {
 
 // Mount CIPHERDIR "c" on PLAINDIR "p"
 // Creates "p" if it does not exist.
+//
+// Contrary to InitFS(), you MUST passt "-extpass=echo test" (or another way for
+// getting the master key) explicitely.
 func Mount(c string, p string, showOutput bool, extraArgs ...string) error {
 	args := []string{"-q", "-wpanic", "-nosyslog", "-fg", fmt.Sprintf("-notifypid=%d", os.Getpid())}
 	args = append(args, extraArgs...)
@@ -93,6 +96,9 @@ func Mount(c string, p string, showOutput bool, extraArgs ...string) error {
 }
 
 // MountOrExit calls Mount() and exits on failure.
+//
+// Contrary to InitFS(), you MUST passt "-extpass=echo test" (or another way for
+// getting the master key) explicitely.
 func MountOrExit(c string, p string, extraArgs ...string) {
 	err := Mount(c, p, true, extraArgs...)
 	if err != nil {
@@ -102,6 +108,9 @@ func MountOrExit(c string, p string, extraArgs ...string) {
 }
 
 // MountOrFatal calls Mount() and calls t.Fatal() on failure.
+//
+// Contrary to InitFS(), you MUST passt "-extpass=echo test" (or another way for
+// getting the master key) explicitely.
 func MountOrFatal(t *testing.T, c string, p string, extraArgs ...string) {
 	err := Mount(c, p, true, extraArgs...)
 	if err != nil {