[docs] Document tools tests workflow (dotnet#101186)

* [docs] Document tools tests workflow

docs/workflow/testing/testing-managed-tools.md

@@ -0,0 +1,45 @@
+# Testing managed tools
+
+There are managed unit and functional tests for a number of tools including the
+compiler for NativeAOT (`ILCompiler`), and the trimmer (`illink`).
+
+## Adding new testsuites
+
+To add a new test suite, create a new `.csproj` with a name that ends in `Tests`, such as:
+`MyTool.Tests.csproj`.  The property `IsTestProject` will be set by the `Directories.Build.props` in
+the repository root.  The property will, in turn, add references to the xunit package and the
+apropriate test runner.
+
+Now add a `ProjectToBuild` item in `eng/Substes.props` to one of the existing subsets, such as
+`clr.toolstests`, or a new subset.
+
+## Adding new testsuites to CI
+
+To run the tests in CI, add a new pipeline or add to an exsiting pipeline such as `CLR_Tools_Tests`
+in `eng/pipelines/runtime.yml`.  Update the trigger condition, perhaps by adding a new set of paths
+to `eng/pipelines/common/evaluate-default-paths.yml` in order to run the tests when the tool source
+or the test sources change.
+
+## Running tests locally
+
+Build and run the tests locally either with
+
+```console
+./build.[sh|cmd] -s clr.toolstests -c [Release|Debug] -build -test
+```
+
+or
+
+```console
+./dotnet.[sh|cmd] test .../MyTool.Tests.csproj -c [Release|Debug]
+```
+
+The `dotnet-test` xunit filter mechanisms work to run a single test or a subset of the tests
+
+```console
+./dotnet.[sh|cmd] test .../MyTool.Tests.csproj -c [Release|Debug] --filter "FullyQualifiedName~MyTest"
+```
+
+The above command runs all tests whose fully-qualified name contains the substring `MyTest`.  See
+[dotnet test - Run selective unit tests](https://learn.microsoft.com/en-us/dotnet/core/testing/selective-unit-tests?pivots=mstest#syntax)
+for the full syntax.