Replace dotnet.config, with global.json

[nohwnd]

dotnet.config is no longer suppported way of configuring mtp, replace it with global.json

Description

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Ensure the PR has required unit tests.

Related issue

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[Copilot]

Pull request overview

Replace deprecated dotnet.config-based test runner configuration with global.json, and enforce the intended runner behavior in integration test execution.

Changes:

• Removed dotnet.config and introduced global.json test.runner configuration.
• Updated InvokeDotnetTest to create/validate a global.json in the working directory to force vstest.
• Added a repository-level global.json test.runner value for Microsoft.Testing.Platform.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
test/Microsoft.TestPlatform.TestUtilities/IntegrationTestBase.cs	Writes/validates global.json in the working directory to force vstest for integration runs.
global.json	Adds test.runner configuration at repo root (set to Microsoft.Testing.Platform).
dotnet.config	Removes deprecated test runner configuration file.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.