tests/conftest: fix log message in LabgridComponent.stop()

The class name was meant to be formated as an f-string, but the string
was not marked accordingly. Fix this by using the lazy evaluation
variant via %s.

Fixes: 373874b ("tests/conftest: move common Exporter functionality into generic LabgridComponent")
Signed-off-by: Bastian Krause <bst@pengutronix.de>
(cherry picked from commit ca8d184)

Author: Bastian-Krause

tests/conftest.py

@@ -55,7 +55,7 @@ def __init__(self, cwd):
         self.reader = None
 
     def stop(self):
-        logging.info("stopping {self.__class__.__name__} pid=%s", self.spawn.pid)
+        logging.info("stopping %s pid=%s", self.__class__.__name__, self.spawn.pid)
 
         # let coverage write its data:
         # https://coverage.readthedocs.io/en/latest/subprocess.html#process-termination