docs: take note on parametrized tests

CONTRIBUTING.md

@@ -222,6 +222,7 @@ Again, the most checks can be performed by `tox` command, though some additional
 * On the other hand, it is allowed to change method signatures, however, it is recommended to add new parameters as optional if possible. Deprecated API can be dropped during major release.
 * Enumerations (`Enum` classes) are allowed and recommended. However, it is recommended to use `StrEnum` class if there are from/to string conversions and `IntEnum` otherwise.
 * `Generator` return type is not allowed. Generator functions must return generic `Iterator` object. Documentation should be described as `Yields`, however, because of pylint checks. Unfortunately, `Iterable` return type is not available for generators also, because of specific `contextlib.contextmanager` case.
+* Tests must not be parametrized. It is better to write separate test, which covers specific line of code rather than testing all over again.
 
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