Fix RedirectInput test flakyness (#7771)

## Summary of changes

We are getting [the following error in
master](https://dev.azure.com/datadoghq/dd-trace-dotnet/_build/results?buildId=190387&view=logs&j=b3c3fdd8-5f52-57f5-d8dc-ef70b96c816b&t=15258d2a-ebbf-52c3-618d-fdfc97db216a):
```
##[error][xUnit.net 00:00:14.01]     Datadog.Trace.Tools.dd_dotnet.ArtifactTests.RunCommandTests.RedirectInput [FAIL]
04:57:34 [ERR] [xUnit.net 00:00:14.01]     Datadog.Trace.Tools.dd_dotnet.ArtifactTests.RunCommandTests.RedirectInput [FAIL]
04:57:34 [DBG] [xUnit.net 00:00:14.30] Datadog.Trace.Tools.dd_dotnet.ArtifactTests: SUCCESS: Datadog.Trace.Tools.dd_dotnet.ArtifactTests.RunCommandTests.EmptyCommand() (0.3031637s)
04:57:34 [DBG] [xUnit.net 00:00:14.30] Datadog.Trace.Tools.dd_dotnet.ArtifactTests: STARTED: Datadog.Trace.Tools.dd_dotnet.ArtifactTests.RunCommandTests.Run()
04:57:35 [DBG]   Failed Datadog.Trace.Tools.dd_dotnet.ArtifactTests.RunCommandTests.RedirectInput [4 s]
04:57:35 [DBG]   Error Message:
04:57:35 [DBG]    System.InvalidOperationException : Cannot process request because the process (7424) has exited.
04:57:35 [DBG]   Stack Trace:
04:57:35 [DBG]      at System.Diagnostics.Process.GetProcessHandle(Int32 access, Boolean throwIfExited)
04:57:35 [DBG]    at System.Diagnostics.Process.Kill()
04:57:35 [DBG]    at Datadog.Trace.Tools.dd_dotnet.ArtifactTests.RunCommandTests.RedirectInput() in D:\a\_work\1\s\tracer\test\Datadog.Trace.Tools.dd_dotnet.ArtifactTests\RunCommandTests.cs:line 103
04:57:35 [DBG]   Standard Output Messages:
04:57:35 [DBG]  Platform: X64
04:57:35 [DBG]  TargetPlatform: X64
04:57:35 [DBG]  Configuration: Release
04:57:35 [DBG]  TargetFramework: net48
04:57:35 [DBG]  .NET Core: False
04:57:35 [DBG]  Native Loader DLL: D:\a\_work\1\s\shared\bin\monitoring-home\win-x64\Datadog.Trace.ClrProfiler.Native.dll
04:57:35 [DBG]  D:\a\_work\1\s\shared\bin\monitoring-home\dd-dotnet.cmd run -- D:\a\_work\1\s\artifacts\bin\Samples.Console\release_net48\Samples.Console.exe  echo
04:57:35 [DBG] 
```

 The CI failure was a race condition in the test cleanup code:
  - The test sends input and reads the echo response
  - The sample app exits after printing the echo (as designed)
  - The test tried to unconditionally kill the process
- Sometimes the process had already exited before Kill() was called,
causing InvalidOperationException

  Thce fix:
1. Added a check for !process.HasExited before attempting to kill the
process
2. Catch an InvalidOperationException for race conditions after testing
process.HasExited
  3. Added process.Dispose() for proper resource cleanup
4. Added a small delay after the echo in the sample app to avoid the
sample program to finish before reading it's output
  
## Reason for change

## Implementation details

## Test coverage

## Other details
<!-- Fixes #{issue} -->


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Author: NachoEchevarria

tracer/test/Datadog.Trace.Tools.dd_dotnet.ArtifactTests/RunCommandTests.cs

@@ -3,6 +3,7 @@
 // This product includes software developed at Datadog (https://www.datadoghq.com/). Copyright 2017 Datadog, Inc.
 // </copyright>
 
+using System;
 using System.Text;
 using System.Threading.Tasks;
 using Datadog.Trace.TestHelpers;
@@ -99,7 +100,20 @@ public void RedirectInput()
         }
         finally
         {
-            process.Kill();
+            // The echo command exits after echoing one line, so the process may have already exited
+            try
+            {
+                if (!process.HasExited)
+                {
+                    process.Kill();
+                }
+            }
+            catch (InvalidOperationException)
+            {
+                // Process exited between the HasExited check and Kill() call
+            }
+
+            process.Dispose();
         }
     }