Fix: Break redirect loop if oauth_auto_login = true and OAuth login fails (#17974)

* Add tests for login view

* Fix OAuth auto login redirect loop

login_error cookie is only set when the OAuth login fails
for some reason. Therefore, the login view should return
immediately if a login_error cookie exists before trying
to login the user using OAuth again.

* Fix test

Use 'index-template' instead of 'index' for testing

* Add some comments

pkg/api/common_test.go

@@ -93,6 +93,26 @@ func (sc *scenarioContext) fakeReqWithParams(method, url string, queryParams map
 	return sc
 }
 
+func (sc *scenarioContext) fakeReqNoAssertions(method, url string) *scenarioContext {
+	sc.resp = httptest.NewRecorder()
+	req, _ := http.NewRequest(method, url, nil)
+	sc.req = req
+
+	return sc
+}
+
+func (sc *scenarioContext) fakeReqNoAssertionsWithCookie(method, url string, cookie http.Cookie) *scenarioContext {
+	sc.resp = httptest.NewRecorder()
+	http.SetCookie(sc.resp, &cookie)
+
+	req, _ := http.NewRequest(method, url, nil)
+	req.Header = http.Header{"Cookie": sc.resp.Header()["Set-Cookie"]}
+
+	sc.req = req
+
+	return sc
+}
+
 type scenarioContext struct {
 	m                    *macaron.Macaron
 	context              *m.ReqContext