[RuntimeAsync] Moving tests from runtimelabs.

[VSadov]

This is a test-only change.

• The tests are currently disabled.
  The easiest way to enable is by removing src\tests\async\Directory.Build.targets, but the build will need to use experimental compiler that understands async2 syntax.
• Benchmarks are included.
  We may still need them in the near future. Will remove later.
• The tests use the old async2 syntax as in the experiment.
  We will have to adjust the syntax once switching to the actual Roslyn prototype compiler with runtime async support.

(see syntax switch Draft PR for details - dotnet/runtimelab#3062)

[Copilot]

Pull Request Overview

This PR moves tests from runtimelabs and introduces a suite of experimental tests using the async2 syntax alongside traditional async usage. Key changes include:

• New test implementations for async method behavior and benchmarks.
• Integration tests covering recursive async calls, Fibonacci benchmarks, and collectible assembly load contexts.
• Mixed usage of async and async2 to validate experimental compiler behavior.

Reviewed Changes

Copilot reviewed 55 out of 66 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/tests/async/implement.cs	Introduces async method implementations with mixed async/async2 usage
src/tests/async/gc-roots-scan.cs	Adds recursive async tests and GC scanning benchmarks
src/tests/async/fibonacci-without-yields.cs	Adds Fibonacci test without yield calls
src/tests/async/fibonacci-without-yields-config-await.cs	Adds Fibonacci test using AsyncHelpers for await configuration
src/tests/async/fibonacci-with-yields_struct_return.cs	Adds Fibonacci test with struct returns (note an apparent naming typo)
src/tests/async/fibonacci-with-yields.cs	Adds Fibonacci test incorporating yield calls
src/tests/async/cse-array-index-byref.cs	Adds test for array index mutation within async methods
src/tests/async/collectible-alc.cs	Adds test for unloading collectible assembly load contexts
src/tests/async/awaitingnotasync.cs	Adds test for intermixing await with non-async methods

Files not reviewed (11)

• src/tests/async/Directory.Build.targets: Language not supported
• src/tests/async/asynctests.csproj: Language not supported
• src/tests/async/awaitingnotasync.csproj: Language not supported
• src/tests/async/collectible-alc.csproj: Language not supported
• src/tests/async/cse-array-index-byref.csproj: Language not supported
• src/tests/async/eh-microbench.csproj: Language not supported
• src/tests/async/fibonacci-with-yields.csproj: Language not supported
• src/tests/async/fibonacci-with-yields_struct_return.csproj: Language not supported
• src/tests/async/fibonacci-without-yields-config-await.csproj: Language not supported
• src/tests/async/fibonacci-without-yields.csproj: Language not supported
• src/tests/async/gc-roots-scan.csproj: Language not supported

Comments suppressed due to low confidence (1)

src/tests/async/fibonacci-with-yields_struct_return.cs:11

• The class name 'Async2FibonacceWithYields' appears to contain a typo. Consider renaming it to 'Async2FibonacciWithYields' for clarity.

public class Async2FibonacceWithYields

[VSadov]

CC: @agocke

The goal is mostly to have everything in one repo.
We can still test the syntax/compiler switching in runtimelabs, if more convenient, and then port over.

[VSadov]

CC: @jakobbotsch

[VSadov]

/ba-g the wasm failures could not be related