Fix Azure Devops Reporter : new change assumes Python 3.6+ (#7438)

* It seems we don't test this codepath in default test runs, and the mentioned changes introduce a python 3.6-ism.  The fix is simply to access the RegEx's groups via the older method in all cases.

* Add expected-to-fail test to exercise failure reporting path

* New approach; specify a failure test result with all the bad characters in it like we do with JUnit and XUnit

src/Microsoft.DotNet.Helix/Sdk/tools/azure-pipelines/reporter/formats/xunit.py

@@ -24,7 +24,7 @@ def bs(match):
         sym = grp[1]
         if sym == 'x':
             return chr(int(grp[2:], 16))
-        return _unescape_char_map.get(match[0][1]) or sym
+        return _unescape_char_map.get(match.group(0)[1]) or sym
     return re.sub(r'\\x[0-9a-fA-F][0-9a-fA-F][0-9a-fA-F]?[0-9a-fA-F]?|\\[^x]', bs, value)
 
 class XUnitFormat(ResultFormat):

tests/UnitTests.proj

@@ -46,6 +46,17 @@
     <XUnitProject Remove="..\src\Microsoft.DotNet.Build.Tasks.VisualStudio.Tests\Microsoft.DotNet.Build.Tasks.VisualStudio.Tests.csproj"/>
   </ItemGroup>
 
+  <ItemGroup>
+    <HelixWorkItem Include="FailingXUnitTestsTest">
+      <Command>echo 'done!'</Command>
+      <PayloadDirectory>$(MSBuildThisFileDirectory)XUnit</PayloadDirectory>
+    </HelixWorkItem>
+  </ItemGroup>
+  <!-- Exercise Helix failure reporting path for Xunit --> 
+  <ItemGroup>
+    <AzurePipelinesExpectedTestFailure Include="XunitReportingTests.ExerciseXunitCharacterFilteringFailurePath"/>
+  </ItemGroup>
+
   <ItemGroup Condition=" '$(HelixAccessToken)' != '' ">
     <HelixTargetQueue Include="Debian.9.Amd64"/>
     <HelixTargetQueue Include="RedHat.7.Amd64"/>

tests/XUnit/testResults.xml

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="utf-8"?>
+<assemblies timestamp="05/24/2021 16:33:36">
+  <assembly name="SomeXUnitTests.DLL" total="1" passed="0" failed="1" skipped="0" time="0.435" errors="0">
+    <errors />
+    <collection total="1" passed="0" failed="1" skipped="0" name="Test collection for XunitReportingTests" time="0.056">
+      <test name="XunitReportingTests.ExerciseXunitCharacterFilteringFailurePath" type="XunitReportingTests" method="ExerciseXunitCharacterFilteringFailurePath" time="0.0564499" result="Fail">
+        <failure exception-type="Xunit.Sdk.TrueException">
+          <message><![CDATA[Intentional Failure 🍕 \r \n \t \a \b \v \f \0\r\nExpected: True\r\nActual:   False]]></message>
+          <stack-trace><![CDATA[   at XunitReportingTests.ExerciseXunitCharacterFilteringFailurePath() in Class1.cs:line 1]]></stack-trace>
+        </failure>
+      </test>
+    </collection>
+  </assembly>
+</assemblies>