Add cache test for admins (#31265)

Add a test to probe the cache similar to the email test func.

![image](700e2733-586d-4091-900f-f5f71e6e94bf)

![image](2a953802-18fc-4e81-a37d-24ebe1297365)

![image](e00d62ad-bb60-41cc-9138-09993daee156)

---------

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: silverwind <me@silverwind.io>
(cherry picked from commit 363c1235987793dffa5cc851aaae585eb81f091e)

Conflicts:
	options/locale/locale_en-US.ini
	templates/admin/self_check.tmpl
	trivial context conflict

routers/web/admin/admin.go

@@ -14,6 +14,7 @@ import (
 	activities_model "code.gitea.io/gitea/models/activities"
 	"code.gitea.io/gitea/models/db"
 	"code.gitea.io/gitea/modules/base"
+	"code.gitea.io/gitea/modules/cache"
 	"code.gitea.io/gitea/modules/graceful"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
@@ -211,6 +212,14 @@ func SelfCheck(ctx *context.Context) {
 
 		ctx.Data["DatabaseCheckHasProblems"] = hasProblem
 	}
+
+	elapsed, err := cache.Test()
+	if err != nil {
+		ctx.Data["CacheError"] = err
+	} else if elapsed > cache.SlowCacheThreshold {
+		ctx.Data["CacheSlow"] = fmt.Sprint(elapsed)
+	}
+
 	ctx.HTML(http.StatusOK, tplSelfCheck)
 }