examples: make examples testable

examples/m2mbidi/README.md

@@ -8,3 +8,9 @@ Each user can **have many** friends. If user A becomes a friend of B, B is also
 ```console
 go generate ./...
 ```
+
+### Run Example
+
+```console
+go test
+```

examples/m2mbidi/example_test.go

@@ -15,7 +15,7 @@ import (
 	_ "github.com/mattn/go-sqlite3"
 )
 
-func main() {
+func Example_M2MBidi() {
 	client, err := ent.Open("sqlite3", "file:ent?mode=memory&cache=shared&_fk=1")
 	if err != nil {
 		log.Fatalf("failed opening connection to sqlite: %v", err)
@@ -29,6 +29,10 @@ func main() {
 	if err := Do(ctx, client); err != nil {
 		log.Fatal(err)
 	}
+	// Output:
+	// [User(id=1, age=30, name=a8m)]
+	// [User(id=2, age=28, name=nati)]
+	// [User(id=1, age=30, name=a8m) User(id=2, age=28, name=nati)]
 }
 
 func Do(ctx context.Context, client *ent.Client) error {