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.
original commit: d50d0f8bcab260f404049029b77407e0222f588a
