Work on integration tests

- add new entries for #70 and #74
- cleaned-up / enriched a little bit compute and exec tests

Signed-off-by: dubo-dubon-duponey <dubodubonduponey+github@pm.me>

examples/tests/exec/undefined/with_pkg_optional/main.cue

@@ -0,0 +1,14 @@
+package testing
+
+import (
+	"dagger.cloud/optional"
+)
+
+#dagger: {
+	compute: [
+		{
+			do: "load",
+			from: optional
+		},
+	]
+}