Cache date in ConfigGenerator unit test

Occasionally we run into timing issues. For example, if the config
is generated at 12:00:59PM and the assertion is at 12:01:00PM, then
the test will fail. This should bring the expected $date closer to
when the config is generated.

Signed-off-by: Jeff Chan <jefftchan@gmail.com>
Merge pull request #1068 from jeffchan/fix-config-generator-test

Cache date in ConfigGenerator unit test