Added gc.collect() calls to avoid spurious test failures

src/ZODB/tests/dbopen.txt

@@ -330,7 +330,7 @@ resources (like RDB connections), for the duration.
     >>> st = Storage()
     >>> db = DB(st, pool_size=2)
     >>> conn0 = db.open()
-    >>> conn0.cacheMinimize() # See fix84.rst
+    >>> conn0.cacheMinimize(); import gc; _ = gc.collect() # See fix84.rst
     >>> len(conn0._cache)  # empty now
     0
     >>> import transaction

src/ZODB/tests/testCache.py

@@ -444,7 +444,7 @@ def test_basic_cache_size_estimation():
     >>> import ZODB.MappingStorage
     >>> db = ZODB.MappingStorage.DB()
     >>> conn = db.open()
-    >>> conn.cacheMinimize() # See fix84.rst
+    >>> conn.cacheMinimize(); _ = gc.collect() # See fix84.rst
 
     >>> def check_cache_size(cache, expected):
     ...     actual = cache.total_estimated_size