Fix dotnet test hang when the test app process exits but a child process of it remains alive

[Youssef1313]

Fixes #51507

[Copilot]

Pull request overview

This PR adds a test case and implementation changes to prevent dotnet test from hanging when a test application exits but leaves a child process alive that has inherited stdout/stderr handles. The fix modifies the Microsoft Testing Platform (MTP) test runner to wait for process exit independently of consuming stdout/stderr, with a 5-second timeout for output consumption after the process has exited.

Changes:

• Modified TestApplication.RunAsync to wait for process exit before reading stdout/stderr, with a 5-second timeout to handle orphaned child processes
• Added WaitForExitWithoutOutputAsync helper method that waits for process exit without blocking on output streams
• Added MTPChildProcessHangTest test project that simulates a test app spawning a hanging child process
• Added a skipped test case to verify the fix (skipped because test infrastructure itself has the same issue)

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/Cli/dotnet/Commands/Test/MTP/TestApplication.cs	Changed stdout/stderr reading to use line-based reading with StringBuilder, implemented WaitForExitWithoutOutputAsync, and added 5-second timeout after process exit
test/dotnet.Tests/CommandTests/Test/GivenDotnetTestBuildsAndRunsTests.cs	Added skipped test case for MTP child process hang scenario
test/TestAssets/TestProjects/MTPChildProcessHangTest/MTPChildProcessHangTest.csproj	Test project configuration for hanging child process scenario
test/TestAssets/TestProjects/MTPChildProcessHangTest/global.json	Configures test to use Microsoft.Testing.Platform runner
test/TestAssets/TestProjects/MTPChildProcessHangTest/Program.cs	Test implementation that spawns a hanging child process during test execution

[Youssef1313]

/backport to release/10.0.3xx

[github-actions]

Started backporting to release/10.0.3xx (link to workflow run)