Tests: switched to using f-strings.

Previously, it was necessary to support older versions of Python for
compatibility.  F-strings were released in Python 3.6.  Python 3.5 was
marked as unsupported by the end of 2020, so now it's possible to start
using f-strings safely for better readability and performance.

test/unit/log.py

@@ -20,4 +20,4 @@ class Log:
         Log.pos[name] = pos
 
     def get_path(name=UNIT_LOG):
-        return Log.temp_dir + '/' + name
+        return f'{Log.temp_dir}/{name}'