Adding tests to monotonic_array.py (TheAlgorithms#12073)

* Contributes to TheAlgorithms#9943 by adding tests to monotonic_array.py

Addeded doctest in the if __name__. Checks for negaitves and an array of same integers

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---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

data_structures/arrays/monotonic_array.py

@@ -9,6 +9,16 @@ def is_monotonic(nums: list[int]) -> bool:
     True
     >>> is_monotonic([1, 3, 2])
     False
+    >>> is_monotonic([1,2,3,4,5,6,5])
+    False
+    >>> is_monotonic([-3,-2,-1])
+    True
+    >>> is_monotonic([-5,-6,-7])
+    True
+    >>> is_monotonic([0,0,0])
+    True
+    >>> is_monotonic([-100,0,100])
+    True
     """
     return all(nums[i] <= nums[i + 1] for i in range(len(nums) - 1)) or all(
         nums[i] >= nums[i + 1] for i in range(len(nums) - 1)
@@ -21,3 +31,7 @@ def is_monotonic(nums: list[int]) -> bool:
     print(is_monotonic([1, 2, 2, 3]))  # Output: True
     print(is_monotonic([6, 5, 4, 4]))  # Output: True
     print(is_monotonic([1, 3, 2]))  # Output: False
+
+    import doctest
+
+    doctest.testmod()