fix test cases on Windows

SysProcAttr.Credential is not available on Windows we need to move the
WrapCmd test in a separate file to be able to build test cases on Windows,
skipping the test is not enough

sftpd/internal_unix_test.go

@@ -0,0 +1,19 @@
+// +build !windows
+
+package sftpd
+
+import (
+	"os/exec"
+	"testing"
+)
+
+func TestWrapCmd(t *testing.T) {
+	cmd := exec.Command("ls")
+	cmd = wrapCmd(cmd, 1000, 1001)
+	if cmd.SysProcAttr.Credential.Uid != 1000 {
+		t.Errorf("unexpected uid")
+	}
+	if cmd.SysProcAttr.Credential.Gid != 1001 {
+		t.Errorf("unexpected gid")
+	}
+}