You just do raw byte copies from sample DB, no SQL or "inserts" or anything similar. Imagine test database consists of 3 parts (all raw bytes)

### PROLOGUE ### Sample row ### EPILOGUE

You copy & write prologue, write 1B sample raws (can optimize this at will, large writes, etc)

Copy & write epilogue and fsync the data. You probably need to modify some metadata, but that should be a few writes at most.

That should be as good as it gets, providing your IO is optimal.