The testing function in export_test.go is changed to just export.go so
that callers outside the ffldb package will be able to call the
function.
The main use for this is so that the prune code can be triggered from
the blockchain package. This allows testing code to have less than
1.5GB worth of blocks to trigger the prune.
This change is part of the effort to add pruning support to btcd.
PruneBlocks will prune the earliest block files until it reaches the
given target size. The returned hashes are the hashes of the blocks
that were pruned.
