refactor: Add remaining tests for 100% coverage in Option

Author: MarcellPerger1

src/test/java/net/marcellperger/mathexpr/util/rs/OptionTest.java

@@ -89,6 +89,16 @@ void mapOrElse() {
         }
     }
 
+    @Test
+    void mapErr() {
+        MockedRunnable mRunnable = new MockedRunnable();
+        assertEquals(getNone(), getNone().mapErr(mRunnable));
+        mRunnable.assertCalledOnce();
+        mRunnable.reset();
+        assertEquals(getSome(), getSome().mapErr(mRunnable));
+        mRunnable.assertNotCalled();
+    }
+
     @Test
     void ifThenElse() {
         MockedSupplier<Integer> mSupplier = new MockedSupplier<>(-6);
@@ -329,4 +339,10 @@ void xor() {
         assertEquals(getSome(), getNone().xor(getSome()));
         assertEquals(getNone(), getNone().xor(getNone()));
     }
+
+    @Test
+    void test_toString() {
+        assertEquals("Some(314)", getSome().toString());
+        assertEquals("None", getNone().toString());
+    }
 }