bachefs-tools: Fix broken test code.

A check was missing in some of the python test code, probably from a bad
cherry-pick.

Signed-off-by: Justin Husted <sigstop@gmail.com>

tests/util.py

@@ -210,6 +210,7 @@ class BFuse:
         run("fusermount3", "-zu", self.mnt)
         print("Waiting for thread to exit.")
 
+        if self.thread:
             self.thread.join(timeout)
             if self.thread.is_alive():
                 self.proc.kill()