How to write a junit test for my mergesort?

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What I have tried:

<pre lang="java"><pre>p

import static org.junit.Assert.assertTrue;
import static org.junit.jupiter.api.Assertions.assertThrows;
import org.junit.jupiter.api.Test;
import de.hska.iwi.ads.sorting.Sort;
import java.util.Arrays;

public class MergesortTest extends SortTest {

	@Override
	public <E extends Comparable<E>> Sort<E> createSort() {
		return new Mergesort<E>();
	}

//Test für Integer
	@Test
	void testMergesortInteger() {
		Integer[] a = { 2, 1, 7, 12, 5, 9, 8, 10 };
		Integer[] result = { 1, 2, 5, 7, 8, 9, 10, 12 };

		Mergesort<Integer> ms = new Mergesort<Integer>();
		ms.sort(a);
		assertTrue(Arrays.equals(a, result));
	}

	@Test
	void testMergesortInteger1() {
		Integer[] a = { 5, 2, 1, 2, 5, 4, 2 };
		Integer[] result = { 1, 2, 2, 2, 4, 5, 5 };

		Mergesort<Integer> ms = new Mergesort<Integer>();
		ms.sort(a);
		assertTrue(Arrays.equals(a, result));
	}

//Test für String
	@Test
	void testMergesortString() {
		String[] a = { "r", "a", "b", "d" };
		String[] result = { "a", "b", "d", "r" };

		Mergesort<String> ms = new Mergesort<String>();
		ms.sort(a);
		assertTrue(Arrays.equals(a, result));
	}

	@Test
	void testMergesortString1() {
		String[] a = { "baby", "a", "king", "to", "music", "blue" };
		String[] result = { "a", "baby", "blue", "king", "music", "to" };

		Mergesort<String> ms = new Mergesort<String>();
		ms.sort(a);
		assertTrue(Arrays.equals(a, result));
	}

//Test für null
	@Test
	void testMergesortIntArrayIntNull() {
		Integer[] a = null;
		Mergesort<Integer> ms = new Mergesort<Integer>();
		assertThrows(NullPointerException.class, () -> ms.sort(a));
	}



//test for one element
	@Test
	void testMergesortOneElement() {
		Integer[] a = { 5 };
		Integer[] result = { 5 };

		Mergesort<Integer> ms = new Mergesort<Integer>();
		ms.sort(a);
		assertTrue(Arrays.equals(a, result));

	}

	// test for two element
	@Test
	void testMergesortTwoElement() {
		Integer[] a = { 5, 3 };
		Integer[] result = { 3, 5 };

		Mergesort<Integer> ms = new Mergesort<Integer>();
		ms.sort(a);
		assertTrue(Arrays.equals(a, result));

	}

	// test for 100.000 Element