Fix random bugs in rendition service tests

Some rendition-related tests are too strict with respect to date comparisons when using database storages that round them, causing failures on MySQL and SQL Server on these tests:

• TestRenditionService.doZipTreeExportLazyRendition
• TestRenditionService.shouldStoreRenditionByDefault

https://qa.nuxeo.org/jenkins/job/master/job/nuxeo-master-fullbuild-part1-multidb-linux/Slave=MULTIDB_LINUX,dbprofile=mysql,jdk=java-8-oracle/203/
https://qa.nuxeo.org/jenkins/job/master/job/nuxeo-master-fullbuild-part1-multidb-linux/Slave=MULTIDB_LINUX,dbprofile=mssql,jdk=java-8-oracle/203/