Update plan tests to use Europa constructs

Signed-off-by: Joel Longtine <joel@dagger.io>

tests/plan/hello-europa/main.cue

@@ -7,11 +7,21 @@ import (
 
 dagger.#Plan & {
 	actions: {
-		// TODO/FIXME: Use Europa constructs for this
-		// sayHello: os.#Container & {
-		//  command: "echo Hello Europa! > /out.txt"
-		// }
+		image: dagger.#Pull & {
+			source: "alpine:3.15.0@sha256:e7d88de73db3d3fd9b2d63aa7f447a10fd0220b7cbf39803c803f2af9ba256b3"
+		}
 
-		// verify: "Hello Europa!\n" & (os.#File & {from: sayHello, path: "/out.txt"}).contents
+		exec: dagger.#Exec & {
+			input: image.output
+			args: ["sh", "-c", "echo -n Hello Europa > /out.txt"]
+		}
+
+		verify: dagger.#ReadFile & {
+			input: exec.output
+			path:  "/out.txt"
+		} & {
+			// assert result
+			contents: "Hello Europa"
+		}
 	}
 }