Add simple test environment for core features

Signed-off-by: Solomon Hykes <solomon@dagger.io>

.dagger/env/test-core/plan/test-core.cue

@@ -0,0 +1,11 @@
+package testcore
+
+import (
+	"dagger.io/dagger"
+)
+
+name: string | *"world" @dagger(input)
+message: "Hello, \(name)!" @dagger(output)
+
+dir: dagger.#Artifact @dagger(input)
+samedir: dir @dagger(output)