Enable code coverage for Linux/Mac

[fhnaseer]

Fixes #1854

Context

Changes Made

Testing

Notes

[Copilot]

Pull Request Overview

This PR implements comprehensive code coverage collection for the MSBuild repository by configuring the test infrastructure to use VSTest runner with Microsoft.CodeCoverage data collector instead of the previous xUnit runner.

Key changes include:

• Switching from xUnit to VSTest runner with code coverage enabled
• Adding runsettings files to configure coverage collection for different platforms
• Creating PowerShell and Bash scripts to process and merge coverage reports
• Updating CI pipeline to collect, process, and publish coverage artifacts from all build jobs
• Adding test isolation improvements to prevent cross-test interference

Reviewed Changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/Directory.Build.targets	Switches test runner from xUnit to VSTest with coverage settings and filters
src/Directory.Build.props	Enables VSTest runner for test projects
CoverageWindowsFull.runsettings	Configures code coverage collection for Windows with specific module filters
Coverage.runsettings	Configures basic code coverage collection for non-Windows platforms
.vsts-dotnet-ci.yml	Updates CI pipeline to collect, process and publish coverage reports
eng/process-coverage.ps1	PowerShell script to merge coverage reports using dotnet-coverage tool
eng/process-coverage.sh	Bash script equivalent for Unix platforms
eng/restore-dotnet-coverage.ps1	PowerShell script to install dotnet-coverage tool
eng/restore-dotnet-coverage.sh	Bash script equivalent for Unix platforms
Various test files	Adds test isolation and timeout improvements

[YuliiaKovalova]

for the record

coverage for windows is quite flaky, sometimes it will finish in 50 minutes but then sometimes it will stuck forever,
so i disabled coverage for it for now, maybe we can fix it later,