Make composition and boundary tests extension-friendly

internal/modules/seriatim/register/register_test.go

@@ -1,7 +1,6 @@
 package register
 
 import (
-	"reflect"
 	"strings"
 	"testing"
 
@@ -13,8 +12,22 @@ func TestRegisterAddsSeriatimFamily(t *testing.T) {
 	if err := Register(registries, nil); err != nil {
 		t.Fatalf("Register() error = %v, want nil", err)
 	}
-	if got, want := registries.Inputs.RegisteredKeys(), []string{"seriatim"}; !reflect.DeepEqual(got, want) {
-		t.Fatalf("input keys = %#v, want %#v", got, want)
+	assertContainsKeys(t, "inputs", registries.Inputs.RegisteredKeys(), []string{"seriatim"})
+	if adapter, err := registries.Inputs.Build("seriatim"); err != nil || adapter.Key() != "seriatim" {
+		t.Fatalf("build seriatim input = %v, %v; want seriatim implementation", adapter, err)
+	}
+}
+
+func assertContainsKeys(t *testing.T, name string, got, want []string) {
+	t.Helper()
+	seen := make(map[string]struct{}, len(got))
+	for _, key := range got {
+		seen[key] = struct{}{}
+	}
+	for _, key := range want {
+		if _, ok := seen[key]; !ok {
+			t.Fatalf("%s keys = %#v, want required key %q", name, got, key)
+		}
 	}
 }