Implemented Test.getAverageScore and Percentage

lib/Test.js

@@ -151,6 +151,19 @@ class Test {
 		}));
 	}
 
+	async getAverageScore() {
+		const trs = await this.getTestResults();
+
+		return trs.reduce((a, b) => a + b.mark, 0) /
+			(trs.length || 1);
+	}
+
+	async getAveragePercentage() {
+		return await this.getAverageScore() /
+			this.template.maxMark *
+			100;
+	}
+
 	async addResult(accountId, studentId, mark) {
 		const tr = await TestResult.create(
 			this.#conn,