Disable a regex test that is failing with minopts (dotnet#19488)

See dotnet/coreclr#18912. This test would always fail once tiered compilation is enabled by default in CoreFX test runs in the CoreCLR repo.

Author: kouvel

tests/CoreFX/CoreFX.issues.json

@@ -239,6 +239,20 @@
             ]
         }
     },
+    {
+        "name": "System.Text.RegularExpressions.Tests",
+        "enabled": true,
+        "exclusions": {
+            "namespaces": null,
+            "classes": null,
+            "methods": [
+                {
+                    "name": "System.Text.RegularExpressions.Tests.RegexMatchTests.Match_ExcessPrefix",
+                    "reason": "https://github.com/dotnet/coreclr/issues/18912"
+                }
+            ]
+        }
+    },
     {
         "name": "System.Threading.Thread.Tests",
         "enabled": true,