refactor(backend): Streamlining van de testdata voor leerpaden en leerobjecten + integratie in seed

Hierbij ook testdata functionaliteit toegevoegd om makkelijk nieuwe leerpaden aan te maken.

backend/tests/test_assets/content/learning-object-resources/dummy/dummy-learning-object-example.ts

@@ -0,0 +1,32 @@
+import { LearningObjectExample } from '../learning-object-example';
+import { LearningObject } from '../../../../../src/entities/content/learning-object.entity';
+import { Language } from '@dwengo-1/common/dist/util/language';
+import { loadTestAsset } from '../../../../test-utils/load-test-asset';
+import { DwengoContentType } from '../../../../../src/services/learning-objects/processing/content-type';
+import { envVars, getEnvVar } from '../../../../../src/util/envVars';
+
+/**
+ * Create a dummy learning object to be used in tests where multiple learning objects are needed (for example for use
+ * on a path), but where the precise contents of the learning object are not important.
+ */
+export function dummyLearningObject(hruid: string, language: Language, title: string): LearningObjectExample {
+    return {
+        createLearningObject: (): LearningObject => {
+            const learningObject = new LearningObject();
+            learningObject.hruid = getEnvVar(envVars.UserContentPrefix) + hruid;
+            learningObject.language = language;
+            learningObject.version = 1;
+            learningObject.title = title;
+            learningObject.description = 'Just a dummy learning object for testing purposes';
+            learningObject.contentType = DwengoContentType.TEXT_PLAIN;
+            learningObject.content = Buffer.from('Dummy content');
+            learningObject.returnValue = {
+                callbackUrl: `/learningObject/${hruid}/submissions`,
+                callbackSchema: '[]',
+            };
+            return learningObject;
+        },
+        createAttachment: {},
+        getHTMLRendering: () => loadTestAsset('learning-objects/dummy/rendering.txt').toString(),
+    };
+}

backend/tests/test_assets/content/learning-object-resources/dummy/rendering.txt

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+Dummy content