enable compilation of tests formerly in "racket-test"; reorganize

A package like "gui" does not depend on a package like "gui-test",
which means that you have to specifically ask for a test package if
you want the tests. A new "main-distribution-test" package
aggregates the tests for packages that are in "main-distribution".

The "plt-services" package, meanwhile, depends on the
"main-distribution-test" package, which means that all tests are
compiled by default for an in-place build.