chore: Refactor test setup for Gin framework

- Removed redundant `gin.SetMode(gin.TestMode)` calls from individual test files.
- Introduced a centralized `TestMain` function in `testmain_test.go` to set the Gin mode for all tests.
- Ensured consistent test environment setup across various handler test files.

backend/internal/api/handlers/crowdsec_dashboard.go

@@ -203,7 +203,7 @@ func (h *CrowdsecHandler) fetchActiveDecisionCount(ctx context.Context) int64 {
 		}
 	}
 
-	baseURL, err := validateCrowdsecLAPIBaseURL(lapiURL)
+	baseURL, err := h.resolveLAPIURLValidator(lapiURL)
 	if err != nil {
 		return -1
 	}
@@ -476,7 +476,7 @@ func (h *CrowdsecHandler) fetchLAPIAlerts(ctx context.Context, since time.Time,
 		}
 	}
 
-	baseURL, err := validateCrowdsecLAPIBaseURL(lapiURL)
+	baseURL, err := h.resolveLAPIURLValidator(lapiURL)
 	if err != nil {
 		return h.fetchAlertsCscli(ctx, scenario, limit)
 	}