Add more info for StdErrAfter assertion in host tests (#118712)

The assertion only ended up printing stderr, but it would be useful to have all the information about the command that was run (the exit code, stdout).

Author: elinor-fung

src/installer/tests/TestUtils/Assertions/CommandResultAssertions.cs

@@ -2,7 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
-using System.Linq;
 using System.Text.RegularExpressions;
 using FluentAssertions;
 using FluentAssertions.Execution;
@@ -154,18 +153,6 @@ public AndConstraint<CommandResultAssertions> NotFileContains(string path, strin
             return new AndConstraint<CommandResultAssertions>(this);
         }
 
-        public string GetDiagnosticsInfo()
-            => $"""
-
-                File Name: {Result.StartInfo.FileName}
-                Arguments: {Result.StartInfo.Arguments}
-                Environment:
-                {string.Join(Environment.NewLine, Result.StartInfo.Environment.Where(i => i.Key.StartsWith(Constants.DotnetRoot.EnvironmentVariable)).Select(i => $"  {i.Key} = {i.Value}"))}
-                Exit Code: 0x{Result.ExitCode:x}
-                StdOut:
-                {Result.StdOut}
-                StdErr:
-                {Result.StdErr}
-                """;
+        public string GetDiagnosticsInfo() => Result.GetDiagnosticsInfo();
     }
 }

src/installer/tests/TestUtils/Assertions/CommandResultExtensions.cs

@@ -4,7 +4,6 @@
 using FluentAssertions;
 using FluentAssertions.Execution;
 using Microsoft.DotNet.Cli.Build.Framework;
-using System;
 
 namespace Microsoft.DotNet.CoreSetup.Test
 {
@@ -20,10 +19,7 @@ public static CommandResult StdErrAfter(this CommandResult commandResult, string
             int i = commandResult.StdErr.IndexOf(pattern);
             i.Should().BeGreaterThanOrEqualTo(
                 0,
-                "Trying to filter StdErr after '{0}', but such string can't be found in the StdErr.{1}{2}",
-                pattern,
-                Environment.NewLine,
-                commandResult.StdErr);
+                $"'{pattern}' should be in StdErr - cannot filter StdErr to after expected string.{commandResult.GetDiagnosticsInfo()}");
             string filteredStdErr = commandResult.StdErr.Substring(i);
 
             return new CommandResult(commandResult.StartInfo, commandResult.ProcessId, commandResult.ExitCode, commandResult.StdOut, filteredStdErr);

src/installer/tests/TestUtils/CommandResult.cs

@@ -2,8 +2,9 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
-using System.Text;
 using System.Diagnostics;
+using System.Linq;
+using System.Text;
 
 namespace Microsoft.DotNet.Cli.Build.Framework
 {
@@ -23,5 +24,19 @@ public CommandResult(ProcessStartInfo startInfo, int pid, int exitCode, string s
             StdOut = stdOut;
             StdErr = stdErr;
         }
+
+        internal string GetDiagnosticsInfo()
+            => $"""
+
+                File Name: {StartInfo.FileName}
+                Arguments: {StartInfo.Arguments}
+                Environment:
+                {string.Join(Environment.NewLine, StartInfo.Environment.Where(i => i.Key.StartsWith("DOTNET_", OperatingSystem.IsWindows() ? StringComparison.OrdinalIgnoreCase : StringComparison.Ordinal)).Select(i => $"  {i.Key} = {i.Value}"))}
+                Exit Code: 0x{ExitCode:x}
+                StdOut:
+                {StdOut}
+                StdErr:
+                {StdErr}
+                """;
     }
 }