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.

original commit: d50d0f8bcab260f404049029b77407e0222f588a

pkgs/gui-pkgs/gui-test/info.rkt

@@ -8,4 +8,9 @@
                "scribble-lib"
                "pconvert-lib"
                "compatibility-lib"))
-(define build-deps '())
+(define build-deps '("draw-lib"
+                     "racket-test"
+                     "sgl"
+                     "snip-lib"
+                     "wxme-lib"
+                     ))