Add cache test for admins (#31265)

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


![image](https://github.com/go-gitea/gitea/assets/24977596/700e2733-586d-4091-900f-f5f71e6e94bf)


![image](https://github.com/go-gitea/gitea/assets/24977596/2a953802-18fc-4e81-a37d-24ebe1297365)


![image](https://github.com/go-gitea/gitea/assets/24977596/e00d62ad-bb60-41cc-9138-09993daee156)

---------

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: silverwind <me@silverwind.io>

modules/cache/cache.go

@@ -4,6 +4,7 @@
 package cache
 
 import (
+	"fmt"
 	"strconv"
 	"time"
 
@@ -35,6 +36,37 @@ func Init() error {
 	return nil
 }
 
+const (
+	testCacheKey       = "DefaultCache.TestKey"
+	SlowCacheThreshold = 100 * time.Microsecond
+)
+
+func Test() (time.Duration, error) {
+	if defaultCache == nil {
+		return 0, fmt.Errorf("default cache not initialized")
+	}
+
+	testData := fmt.Sprintf("%x", make([]byte, 500))
+
+	start := time.Now()
+
+	if err := defaultCache.Delete(testCacheKey); err != nil {
+		return 0, fmt.Errorf("expect cache to delete data based on key if exist but got: %w", err)
+	}
+	if err := defaultCache.Put(testCacheKey, testData, 10); err != nil {
+		return 0, fmt.Errorf("expect cache to store data but got: %w", err)
+	}
+	testVal, hit := defaultCache.Get(testCacheKey)
+	if !hit {
+		return 0, fmt.Errorf("expect cache hit but got none")
+	}
+	if testVal != testData {
+		return 0, fmt.Errorf("expect cache to return same value as stored but got other")
+	}
+
+	return time.Since(start), nil
+}
+
 // GetCache returns the currently configured cache
 func GetCache() StringCache {
 	return defaultCache