tests: replace "naked except clause" to avoid check-code.py error

This is a part of preparation to apply checking with check-code.py on
code fragments embedded in *.t test scripts.

tests/test-acl.t

@@ -38,7 +38,7 b''
   > def fakegetusers(ui, group):
   >     try:
   >         return acl._getusersorig(ui, group)
-  >     except:
+  >     except BaseException:
   >         return [b"fred", b"betty"]
   > acl._getusersorig = acl._getusers
   > acl._getusers = fakegetusers