tests: Fix test timeout error for Python 2.4 on CentOS 5.5

On CentOS 5.5, with Python 2.4 with see sometimes error on node group
cache tests due to sleep issue. With this version, 0.2 sleep time was in
fact 0.19 sleep time and test was too short.

This patch fixes this using a slightly longer wait time.

Change-Id: I81f0b1c06cb292f5c3252bdcfc0b40117fd1b5bf