Add regression test for pylint-dev#8109

Author: Pierre-Sassoulas

tests/functional/r/regression_02/regression_8109.py

@@ -0,0 +1,15 @@
+"""Regression test for https://github.com/PyCQA/pylint/issues/8109."""
+
+# pylint: disable=missing-docstring, unsupported-binary-operation
+
+from dataclasses import dataclass
+
+
+@dataclass
+class Number:
+    amount: int | float
+    round: int = 2
+
+    def __str__(self):
+        number_format = "{:,.%sf}" % self.round  # [consider-using-f-string]
+        return number_format.format(self.amount).rstrip("0").rstrip(".")

tests/functional/r/regression_02/regression_8109.txt

@@ -0,0 +1 @@
+consider-using-f-string:14:24:14:34:Number.__str__:Formatting a regular string which could be a f-string:UNDEFINED