Tests/add new test case weight_conversion (TheAlgorithms#11468)

* add new test

* add new test

Author: ShiryuReb

conversions/weight_conversion.py

@@ -297,6 +297,12 @@ def weight_conversion(from_type: str, to_type: str, value: float) -> float:
     1.660540199e-23
     >>> weight_conversion("atomic-mass-unit","atomic-mass-unit",2)
     1.999999998903455
+    >>> weight_conversion("slug", "kilogram", 1)
+    Traceback (most recent call last):
+    ...
+    ValueError: Invalid 'from_type' or 'to_type' value: 'slug', 'kilogram'
+    Supported values are: kilogram, gram, milligram, metric-ton, long-ton, short-ton, \
+pound, stone, ounce, carrat, atomic-mass-unit
     """
     if to_type not in KILOGRAM_CHART or from_type not in WEIGHT_TYPE_CHART:
         msg = (