darwin: tests/defaults: fix unix.Getdents build failure

Error was:

+ go test -c -tags without_openssl -o /dev/null github.com/rfjakob/gocryptfs/v2/tests/defaults
Error: tests/defaults/main_test.go:532:17: undefined: unix.Getdents
Error: tests/defaults/main_test.go:538:16: undefined: unix.Getdents
Error: tests/defaults/main_test.go:549:16: undefined: unix.Getdents

3 files changed, 23 insertions, 3 deletions

diff --git a/tests/defaults/getdents_linux.go b/tests/defaults/getdents_linux.go
new file mode 100644
index 0000000..57956ce
--- /dev/null
+++ b/tests/defaults/getdents_linux.go
@@ -0,0 +1,9 @@
+package defaults
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+func getdents(fd int, buf []byte) (int, error) {
+	return unix.Getdents(fd, buf)
+}
diff --git a/tests/defaults/getdents_other.go b/tests/defaults/getdents_other.go
new file mode 100644
index 0000000..c0552a8
--- /dev/null
+++ b/tests/defaults/getdents_other.go
@@ -0,0 +1,11 @@
+//go:build !linux
+
+package defaults
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+func getdents(fd int, buf []byte) (int, error) {
+	return unix.Getdirentries(fd, buf, nil)
+}
diff --git a/tests/defaults/main_test.go b/tests/defaults/main_test.go
index 1cc7cb6..75a5dae 100644
--- a/tests/defaults/main_test.go
+++ b/tests/defaults/main_test.go
@@ -529,13 +529,13 @@ func TestSeekDir(t *testing.T) {
 	defer syscall.Close(fd)
 
 	buf := make([]byte, 1000)
-	n, err := unix.Getdents(fd, buf)
+	n, err := getdents(fd, buf)
 	if err != nil {
 		t.Fatal(err)
 	}
 	t.Logf("1st getdents returned %d bytes", n)
 
-	n, err = unix.Getdents(fd, buf)
+	n, err = getdents(fd, buf)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -546,7 +546,7 @@ func TestSeekDir(t *testing.T) {
 		t.Error(err)
 	}
 
-	n, err = unix.Getdents(fd, buf)
+	n, err = getdents(fd, buf)
 	if err != nil {
 		t.Fatal(err)
 	}