This is consistent with the way BackRest and BackRest test were renamed way back in 18fd2523.
More modules will be moving to pgBackRestDoc soon so renaming now reduces churn later.
This directory was once the home of the production Perl code but since f0ef73db this is no longer true.
Move the modules to test in most cases, except where the module is expected to be useful for the doc engine beyond the expected lifetime of the Perl test code (about a year if all goes well).
The exception is pgBackRest::Version which requires more work to migrate since it is used to track pgBackRest versions.
The tests and documentation have been using the core storage layer but soon that will depend entirely on the C library, creating a bootstrap problem (i.e. the storage layer will be needed to build the C library).
Create a simplified Posix storage layer to be used by documentation and the parts of the test code that build and execute the actual tests. The actual tests will still use the core storage driver so they can interact with any type of storage.
