Disable outdates StringBuilder test

tests/TopN.CoreFX.Windows.issues.json

@@ -876,6 +876,10 @@
             "namespaces": null,
             "classes": null,
             "methods": [
+                {
+                    "name": "System.Text.Tests.StringBuilderTests.Equals",
+                    "reason": "Because of a change in StringBuilder, this test is outdated."
+                },
                 {
                     "name": "System.Reflection.Tests.MethodInfoTests.TestEquality2",
                     "reason": "Xunit.Sdk.EqualException"