Triple Nested Loop

Create a function triple_nested_loop that returns a string similar to this one:

....1
...22
..333
.4444
55555

To do it, you'll have to nest 2 for loops inside a bigger/outer for loop. You'll end up with something like this:

for i in range(XXX):
    for j in range(YYY):
        pass
    for m in range(ZZZ):
        pass

The triple_nested_loop should receive a single parameter size. Example:

triple_nested_loop(3)
"""
..1
.22
333
"""

triple_nested_loop(6)
"""
.....1
....22
...333
..4444
.55555
666666
"""
import unittest

EXPECTED_DEFAULT = """
....1
...22
..333
.4444
55555
""".lstrip()

EXPECTED_SMALLER = """
.1
22
""".lstrip()

EXPECTED_LARGER = """
......1
.....22
....333
...4444
..55555
.666666
7777777
""".lstrip()

class TripleNestedLoopTestCase(unittest.TestCase):

    def test_default_parameters(self):
        self.assertEqual(triple_nested_loop(), EXPECTED_DEFAULT)

    def test_triple_nested_smaller_number(self):
        self.assertEqual(
            triple_nested_loop(2), EXPECTED_SMALLER)

    def test_triple_nested_larger_number(self):
        self.assertEqual(
            triple_nested_loop(7), EXPECTED_LARGER)