[acceptance-tests] Integrate Roslyn, CoreCLR and ms-test-suite into acceptance-tests

[acceptance-tests] Integrate Roslyn, CoreCLR and ms-test-suite into acceptance-tests

This commit integrates the CoreCLR, Roslyn and ms-test-suite (which is Xamarin-internal, but should just print a warning for someone without access).
Running the tests suites can be done with 'make check-coreclr', 'make check-roslyn' and 'make check-ms-test-suite', or all of them with make check in the acceptance-tests folder.
They won't run on a top-level make check though, which is intended until every test suite is fixed.

This uses a custom submodule system that doesn't rely on git submodules (which are annoying when the repo is large and we only need them for tests). Instead we can fetch repositories on demand.

History (before squashing into smaller chunks):

- Rename check-{testsuite} make target to validate-{testsuite}. The former can be easily confused with the "check" target that runs the tests.
- Move acceptance-tests Makefile to automake, so we can avoid hardcoding paths
- Copy System.Console facade to CoreCLR IL test directories. Otherwise most of the IL tests fail as they rely on the System.Console assembly reference.
- Update README.md to mention the tests
- Use $(MAKE) instead of plain make and pass -C instead of cd'ing to directory
- Exit when the repository can't be cloned and print a note about the ms-test-suite repo. The ms-test-suite repo is Xamarin-internal so it'll error out in public.
- Integrate MS CoreCLR runtime tests into Mono's test infrastructure
- Get rid of intermediate msg file and pipe straight to git
- Rename README to CONFIG since we don't store metadata in readme.md anymore
- Fix comparison in versions.mk to be sh compliant. It relied on bashism before.