refactor: auto get client ip from challenge response (#5)

* refactor: auto get client ip from challenge response
Since `outip()` was not working properly on devices getting local IP addresses behind a router, we should refactor this.
After analyzing the auth process, it is shown that  the challenge response includes key `client_ip` which is the real public IP address with key `ip` not specified in request.
- removed `outip()`
- added rsp key `ClientIP` to get client ip from challenge rsp

* style: trim code

* style: fix spelling issues

* refactor: create `portal_test.go` to handle portal tests separately

* feature: add `ResolveLocalClientIP()` and its test case

* optimize: resolve ClientIP locally when cant be get from challenge response

portal/portal_test.go

@@ -0,0 +1,24 @@
+package portal
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestAutoSelectServerIP(t *testing.T) {
+	u, err := NewPortal("2000010101001", "12345678", "", "1.2.3.4", LoginTypeQshEdu)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Log(u.sip)
+	assert.Equal(t, PortalServerIPQsh, u.sip)
+}
+
+func TestResolveLocalClientIP(t *testing.T) {
+	cip, err := ResolveLocalClientIP()
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Log(cip)
+}