nixos/test-driver: more context when step finishes

When displaying the amount of time some step took, with no other
context, it becomes nigh impossible (especially in longer tests) to see
when specific steps finished.

(cherry picked from commit c6ee63259a)

nixos/lib/test-driver/test-driver.py

@@ -171,7 +171,7 @@ class Logger:
         yield
         self.drain_log_queue()
         toc = time.time()
-        self.log("({:.2f} seconds)".format(toc - tic))
+        self.log("(finished: {}, in {:.2f} seconds)".format(message, toc - tic))
 
         self.xml.endElement("nest")