test: re-enable undefined prop test

Rather than checking for failure, we verify that the computable part of
the configuration gets outputted.

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

examples/tests/compute/invalid/undefined_prop/main.cue

@@ -1,18 +0,0 @@
-package testing
-
-bar: string
-
-#dagger: {
-	compute: [
-		{
-			do: "fetch-container"
-			ref: "alpine"
-		},
-		{
-			do: "exec"
-			dir: "/"
-			args: ["sh", "-c", "echo \(foo.bar)"]
-		}
-	]
-	foo: bar: bar
-}