Fix random ConfigurationMarshallerTest.testPersistNuxeoConf*

These tests are random in our CI:

• testPersistNuxeoConfConfiguratorParameters
• testPersistNuxeoConfUniqueParameters
• testPersistNuxeoConfNoTemplatesParameter
• testPersistNuxeoConfCommentedParameters

The last modified time of persisted nuxeo.conf are sometime the same, the whole test class is executed in less than 30ms, and writing the file happens in less than a nanosecond. This is probably due to high performance of the FS and CPU on the CI.

We want to remove the assertion on the last modified time and only leverage the file content assertion, this should be enough as we change the content.