Fixed relative CLOB file path bug

Resolving path by changeLog.getPhysicalFilePath rather than by using changeSet.getFilePath

This makes the behavior consistent with the lookup of change logs in change log imports. I've also adapted the mocks in the unit tests.

Additionally, I've created an integration test in the liquibase-maven-plugin module, but I'll submit this in a separate pull request as it uses a Derby in memory DB and I'm not sure if you'd want to have this in your tests.