Fix preview offer check to only flag offers ending with -preview

Co-authored-by: psah434 <114955590+psah434@users.noreply.github.com>

Author: Copilot

arm-ttk/testcases/deploymentTemplate/Virtual-Machines-Should-Not-Be-Preview.test.ps1

@@ -26,7 +26,7 @@ foreach ($sp in $storageProfiles) {
     if ($storageProfile.imageReference -like '*preview' -or `
         $storageProfile.imageReference.version -like '*preview' -or `
         $storageProfile.imageReference.sku -like '*preview' -or `
-        $storageProfile.imageReference.offer -like '*preview*') {
+        $storageProfile.imageReference.offer -like '*-preview') {
         Write-Error "StorageProfile for resource '$($sp.ParentObject.Name)' must not use a preview version" -TargetObject $sp -ErrorId VM.Using.Preview.Image
     }
 }

unit-tests/Virtual-Machines-Should-Not-Be-Preview/Fail/Latest-Preview-offer.json

@@ -9,7 +9,7 @@
                     "imageReference": {
                         "publisher": "Canonical",
                         "version": "latest",
-                        "offer": "UbuntuServer_preview",
+                        "offer": "UbuntuServer-preview",
                         "sku": "16.04-LTS"
                     }
                 }

unit-tests/Virtual-Machines-Should-Not-Be-Preview/Pass/offer-with-preview-in-name.json

@@ -0,0 +1,19 @@
+{
+    "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
+    "resources": [
+        {
+            "type": "Microsoft.Compute/virtualMachines",
+            "apiVersion": "2019-12-01",
+            "properties": {
+                "storageProfile": {
+                    "imageReference": {
+                        "publisher": "TrendMicro",
+                        "version": "latest",
+                        "offer": "zero-trust-secure-access-preview-st",
+                        "sku": "standard"
+                    }
+                }
+            }
+        }
+    ]
+}