Add support for testing via ILLINK

Add support for testing via ILLINK

Add support for testing via ILLINK

This commit has the following changes:
1) Setup the testing infrastructure to run the CoreCLR tests after linking through ILLinker
    `runtest.cmd /link <path-to-illink.exe>`
2) The linker is taken as an argument for runtest.cmd, typically the corebuild of
    https://github.com/mono/linker
3) Tests are run by linking the test binaries and all libraries except `System.Private.Corelib.dll`
4) For tests that use reflection, setup Reflection roots in source directories as `<test>.reflect.xml`

Commit migrated from https://github.com/dotnet/coreclr/commit/7f8ba1a8bd20ce2330b8d733f1e51959470fdd8f