Add test for external assembly probe mechanism

[elinor-fung]

• Only try to convert to a loaded layout by file mapping for images that are files
• Add test that uses corerun with external assembly probe

Contributes to #112706

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PR Overview

This PR adds a new test to verify the external assembly probe mechanism by checking that the TRUSTED_PLATFORM_ASSEMBLIES property is empty.

• Adds a new test in ExternalAssemblyProbe.cs that asserts the runtime does not rely on the TPA mechanism.
• Skips the test on Mono due to unimplemented functionality in the host-runtime contract.

Reviewed Changes

File	Description
src/tests/Loader/ExternalAssemblyProbe/ExternalAssemblyProbe.cs	New test file that checks the empty state of TRUSTED_PLATFORM_ASSEMBLIES

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

[AaronRobinsonMSFT]

Thank you!

[elinor-fung]

/ba-g Failure is #113025