MDL-43835 unit testing: Create unique starting id's for sequences.

When all starting autoincrement ids are the same, it's difficult to detect coding and testing
errors that use the incorrect id in test calls.  The classic case is cmid vs instance id.
To reduce the chance of the coding error we start sequences at different values where possible.

OUBlog installation is the case that highlighted this as it inserts a sitewide course module
at install time and cmid <> instance id in most cases.