tests: Don't try to access db.storage when automatically closing connections

DB.close() does `del self.storage`.

https://github.com/zopefoundation/ZODB/blob/5.6.0-14-g0eae10cd0/src/ZODB/DB.py#L646

This way if DB was closed, but some conn(s) were not, it will crash in
teardown as e.g. below:

    _____________ ERROR at teardown of test_bigfile_zblk1_zdata_reuse ______________

        def teardown_module():
    >       testdb.teardown()

    bigfile/tests/test_filezodb.py:58:
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    self = <wendelin.lib.testing.TestDB_ZEO object at 0x7fb9c0216350>

        def teardown(self):
            # close connections that test code forgot to close
            for connref, tb in self.connv:
                conn = connref()
                if conn is None:
                    continue
                if not conn.opened:
                    continue    # still alive, but closed
                print("W: testdb: teardown: %s left not closed by test code"
                      "; opened by:\n%s" % (conn, tb), file=sys.stderr)

                db = conn.db()
    >           stor = db.storage
    E           AttributeError: 'DB' object has no attribute 'storage'

    lib/testing.py:217: AttributeError

The fix is simple - don't use db.storage at all, because it is not actually used in that code.

lib/testing.py

 # Wendelin. Testing utilities
-# Copyright (C) 2014-2019  Nexedi SA and Contributors.
+# Copyright (C) 2014-2020  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -214,7 +214,6 @@ class TestDB_Base(object):
                   "; opened by:\n%s" % (conn, tb), file=sys.stderr)
 
             db = conn.db()
-            stor = db.storage
             conn.close()
             # DB.close() should close underlying storage and is noop when
             # called the second time.