Fix fuzzy test reports.tests.test_tasks:SendReportDigestTests.test_base

- test_base was using NGReportFactory to create two reports.
- The factory created reports between 1 Jan 2013 and now().
- The test mocked date to 1 Jan 2014
- If factory picked dates after 1 Jan 2014 for the reports the test
  failed.

This commit fixes this error and also makes the assertions stronger.