tests: Explicitly close ZODB connections for places with warnings found by previous patch

bigfile/tests/test_filezodb.py ........W: testdb: teardown: <Connection at 7f8fe2b43b90> left not closed by test code; opened by:
  ...
  File "/home/kirr/src/wendelin/wendelin.core/bigfile/tests/test_filezodb.py", line 754, in test_bigfile_zblk1_zdata_reuse
    _test_bigfile_zblk1_zdata_reuse()
  File "/home/kirr/src/wendelin/wendelin.core/bigfile/tests/test_filezodb.py", line 759, in _test_bigfile_zblk1_zdata_reuse
    root = dbopen()
  File "/home/kirr/src/wendelin/wendelin.core/bigfile/tests/test_filezodb.py", line 47, in dbopen
    return testdb.dbopen()
  File "/home/kirr/src/wendelin/wendelin.core/lib/testing.py", line 188, in dbopen
    self.connv.append( (weakref.ref(conn), ''.join(traceback.format_stack())) )

lib/tests/test_zodb.py .W: testdb: teardown: <Connection at 7f8fe26f13d0> left not closed by test code; opened by:
  ...
  File "/home/kirr/src/wendelin/wendelin.core/lib/tests/test_zodb.py", line 49, in test_deactivate_btree
    root = dbopen()
  File "/home/kirr/src/wendelin/wendelin.core/lib/tests/test_zodb.py", line 30, in dbopen
    return testdb.dbopen()
  File "/home/kirr/src/wendelin/wendelin.core/lib/testing.py", line 188, in dbopen
    self.connv.append( (weakref.ref(conn), ''.join(traceback.format_stack())) )

bigfile/tests/test_filezodb.py

@@ -790,3 +790,5 @@ def _test_bigfile_zblk1_zdata_reuse():
     assert len(zdata_v1) == len(zdata_v2)
     for i in range(len(zdata_v1)):
         assert zdata_v1[i] is zdata_v2[i]
+
+    dbclose(root)

lib/tests/test_zodb.py

@@ -17,7 +17,7 @@
 #
 # See COPYING file for full licensing terms.
 # See https://www.nexedi.com/licensing for rationale and options.
-from wendelin.lib.zodb import deactivate_btree
+from wendelin.lib.zodb import deactivate_btree, dbclose
 from wendelin.lib.testing import getTestDB
 from persistent import Persistent, UPTODATE, GHOST
 from BTrees.IOBTree import IOBTree
@@ -81,3 +81,5 @@ def test_deactivate_btree():
         for obj in [B] + leafv:
             assert obj._p_state == GHOST
             assert obj not in cached
+
+    dbclose(root)