Added task Nested Loops

Author: sofiiako

Loops/Nested for Loop/__init__.py

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+type: edu
+files:
+- name: task.py
+  visible: true
+  placeholders:
+  - offset: 61
+    length: 31
+    placeholder_text: '# TODO: use a nested loop here.'
+- name: tests/test_task.py
+  visible: false
+- name: __init__.py
+  visible: false
+- name: tests/__init__.py
+  visible: false

Loops/Nested for Loop/task-info.yaml

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+## Nested Loops
+
+A nested loop is a loop inside another loop.
+The inner loop is executed once for each iteration of the outer loop.
+
+```python
+adjectives = ["black", "stylish", "expensive"]
+clothes = ["jacket", "shirt", "boots"]
+
+for x in adjectives:
+  for y in clothes:
+    print(x, y)
+```
+Output:
+```text
+black jacket
+black shirt
+black boots
+stylish jacket
+stylish shirt
+stylish boots
+expensive jacket
+expensive shirt
+expensive boots
+```
+<details>
+
+Note that any type of loop can be nested inside another loop. 
+For example, a [`while` loop](course://Loops/While loop) (see further) can be nested inside a `for` loop or vice versa.
+</details>
+
+
+You are given a tic-tac-toe board of 3x3, your task is to print every position. Coordinates along each side
+are stored in the list `coordinates`. The output should be:
+```text
+1 x 1
+1 x 2
+1 x 3
+2 x 1
+...
+```
+and so on for every combination of coordinates.
+
+<div class="hint">
+
+In the nested `for` loop, iterate over the same list once again but using another variable name
+this time (`coordinate2`).
+</div>

Loops/Nested for Loop/task.md

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+coordinates = [1, 2, 3]
+
+for coordinate1 in coordinates:
+    for coordinate2 in coordinates:
+        print(f'{coordinate1} x {coordinate2}')

Loops/Nested for Loop/task.py

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+import unittest
+import contextlib
+import io
+
+f = io.StringIO()
+
+with contextlib.redirect_stdout(f):
+    from task import *
+output = f.getvalue().split('\n')
+
+
+class TestCase(unittest.TestCase):
+    def test_list(self):
+        expected_output = ['1 x 1', '1 x 2', '1 x 3', '2 x 1', '2 x 2', '2 x 3', '3 x 1', '3 x 2', '3 x 3', '']
+        self.assertEqual(expected_output, output, msg='Your result does not match the expected.')