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/undefined_prop/main.cue

@@ -0,0 +1,20 @@
+package testing
+
+hello: "world"
+
+bar: string
+
+#dagger: {
+	compute: [
+		{
+			do: "fetch-container"
+			ref: "alpine"
+		},
+		{
+			do: "exec"
+			dir: "/"
+			args: ["sh", "-c", "echo \(foo.bar)"]
+		}
+	]
+	foo: bar: bar
+}