checkCrossDBInvalidations: make this willing to wait longer in all,

and increase the sleep time per failing loop iteration.  The hope is
this will allow the test to pass on slower and/or "too busy" machines.
I can't test that, though, since I couldn't get the test to fail at
all on a Windows or a Linux box.

NEWS.txt

@@ -40,6 +40,13 @@ Error reporting
   error (for example, a corrupt database can cause this), the message it
   produces no longer contains unprintable characters.
 
+Tests
+-----
+
+- (3.4c2) ``checkCrossDBInvalidations`` suffered spurious failures too often
+  on slow and/or busy machines.  The test is willing to wait longer for
+  success now.
+
 
 What's new in ZODB3 3.4b1?
 ==========================

src/ZEO/tests/ConnectionTests.py

@@ -651,10 +651,13 @@ class ConnectionTests(CommonSetupTearDown):
         transaction.commit()
 
         # Make sure the invalidation is received in the other client.
-        for i in range(10):
+        # We've had problems with this timing out on "slow" and/or "very
+        # busy" machines, so we increase the sleep time on each trip, and
+        # are willing to wait quite a long time.
+        for i in range(20):
             if r1._p_state == -1:
                 break
-            time.sleep(0.1)
+            time.sleep(i / 10.0)
         self.assertEqual(r1._p_state, -1) # ghost
 
         r1.keys() # unghostify