Should we decorate the new files with @requires('largefile', '...')? Documentation:

    largefile - It is okay to run some test that may create huge
                files.  These tests can take a long time and may
                consume >2 GiB of disk space temporarily.

It may be interesting to add an option to regrtest to specify how much space can be used by tests, an option similar to -M/--memlimit which is for memory.

I don't recall if largefile is enabled by default on buildbots or not :-p