[docs] Update mono workflow doc to use `clr.hosts` subset for working with runtime tests

[lambdageek]

It should now be enough to just build clr.tools and clr.hosts as dependencies of the src/tests/build.sh script in order to generate Core_Root.

Update the workflow docs to show how to build and run runtime tests using Mono without building all of CoreCLR.

The docs are in https://github.com/dotnet/runtime/blob/main/docs/workflow/testing/mono/testing.md

Contributes to #58266

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Issue Details

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Author:	lambdageek
Assignees:	-
Labels:	untriaged, area-Infrastructure-mono
Milestone:	-

[lambdageek]

/cc @naricc