Rename racket-extra-test to racket-test-extra.
Split the core racket tests into the "racket-test-core" pkg.

This supports running the core tests on systems which can't run
or don't want to install all the dependencies of the "racket-test"
package.

The "racket-test-core" pkg stil depends on "sandbox-lib" because
the sandbox tests live with the core tests. Hopefully I'll be able
to fix that eventually.
Add configuration for AppVeyor, a Windows CI service.

This configuration will build and test the core of Racket on every
commit. The time constraints are such that only a small portion of
Racket can be built and tested.