Alerting: Fix HCL export for alerts with non-zero "for" field. (#76739)

* Alerting: Fix HCL export for alerts with non-zero "for" field.

Fixes #76734

* fix tests

---------

Co-authored-by: Yuri Tseretyan <yuriy.tseretyan@grafana.com>

pkg/services/ngalert/api/test-data/post-rulegroup-101-export.hcl

@@ -44,6 +44,7 @@ resource "grafana_rule_group" "rule_group_0000" {
 
     no_data_state  = "NoData"
     exec_err_state = "Alerting"
+    for            = "2m"
     is_paused      = false
   }
   rule {

pkg/services/ngalert/api/test-data/post-rulegroup-101-export.json

@@ -64,7 +64,7 @@
           ],
           "noDataState": "NoData",
           "execErrState": "Alerting",
-          "for": "0s",
+          "for": "2m",
           "isPaused": false
         },
         {
@@ -114,4 +114,4 @@
       ]
     }
   ]
-}
+}

pkg/services/ngalert/api/test-data/post-rulegroup-101-export.yaml

@@ -48,7 +48,7 @@ groups:
                 type: math
           noDataState: NoData
           execErrState: Alerting
-          for: 0s
+          for: 2m
           isPaused: false
         - title: reduced testdata query - 2
           condition: B

pkg/services/ngalert/api/test-data/post-rulegroup-101.json

@@ -3,6 +3,7 @@
   "interval": "10s",
   "rules": [
     {
+      "for": "2m",
       "grafana_alert": {
         "title": "prom query with SSE - 2",
         "condition": "condition",