Use generic offer names in test examples instead of vendor-specific names

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

Author: Copilot

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

@@ -7,10 +7,10 @@
             "properties": {
                 "storageProfile": {
                     "imageReference": {
-                        "publisher": "Canonical",
+                        "publisher": "TestPublisher",
                         "version": "latest",
-                        "offer": "UbuntuServer-preview",
-                        "sku": "16.04-LTS"
+                        "offer": "test-offer-preview",
+                        "sku": "standard"
                     }
                 }
             }

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

@@ -7,9 +7,9 @@
             "properties": {
                 "storageProfile": {
                     "imageReference": {
-                        "publisher": "TrendMicro",
+                        "publisher": "TestPublisher",
                         "version": "latest",
-                        "offer": "zero-trust-secure-access-preview-st",
+                        "offer": "product-preview-edition",
                         "sku": "standard"
                     }
                 }