add: one more unit test

server/map_handler/lake_relations/all_lake_names.json

 [
   "Mjøsa",
-  "Skumsjøen"
+  "Skumsjøen",
+  "test_lake"
 ]
\ No newline at end of file

server/map_handler/lake_relations/test_lake_div.json

+{"test-field1": "test-value1", "test-field2": ["test-value2", "testvalue3"]}
\ No newline at end of file

server/map_handler/unit_tests/test_add_new_lake.py

+import json
 from shapely.geometry import Polygon, LineString
 
-from server.map_handler.add_new_lake import create_grid
+from server.consts import LAKE_RELATIONS_PATH
+from server.map_handler.add_new_lake import create_grid, write_json_to_file
 
 
-def test_create_grid_default():
+def test_create_grid_default() -> None:
     # Create a simple square polygon
     test_poly = Polygon([(0, 0), (1, 0), (1, 1), (0, 1)])
 
     # Define the expected grid output
     coordinates = [[(0, 0), (0, 1)], [(0.5, 0), (0.5, 1)], [(1, 0), (1, 1)],
-                [(0, 0), (1, 0)], [(0, 0.4), (1, 0.4)], [(0, 0.8), (1, 0.8)]]
+                   [(0, 0), (1, 0)], [(0, 0.4), (1, 0.4)], [(0, 0.8), (1, 0.8)]]
     expected = [LineString(coords) for coords in coordinates]
 
     grid = create_grid(test_poly, 0.5, 0.4)
     assert grid == expected
+
+
+def test_write_json_to_file() -> None:
+    # Define some test data
+    test_lake_name = "test_lake"
+    test_data = {
+        'test-field1': 'test-value1',
+        'test-field2': [
+            "test-value2",
+            "testvalue3"
+        ],
+    }
+
+    # Call the function that is being tested
+    write_json_to_file(test_lake_name, test_data)
+
+    # Try to read the data from the newly created file
+    with open(LAKE_RELATIONS_PATH + '/' + test_lake_name + '_div.json', 'r') as f:
+        result = json.load(f)
+
+    assert result == test_data
+
+
+
+
+