Work around flaky partitions test with a sleep (#7601)

Author: eddyashton

tests/partitions_test.py

@@ -377,6 +377,10 @@ def set_certs(from_days_diff, validity_period_days, nodes):
     # Restore connectivity with primary, an election may or may not happen
     network.wait_for_primary_unanimity(min_view=r.view + 1)
 
+    # Dropped partition, and even primary unanimity, do not mean node connectivity has been instantaneously restored.
+    # Sleep through potential delays in reconnection.
+    time.sleep(3)
+
     # Set valid node certs so that future clients can speak to these nodes
     set_certs(from_days_diff=-1, validity_period_days=7, nodes=(primary, backup_a))