* test: improve coverage in tr_bitfield::raw()
* test: improve coverage in tr_bitfield::count()
* test: improve coverage for edge cases in tr_bitfield::setSpan()
* test: confirm that excess bits in setRaw() are set to zero
* fix: edge case of tr_block_info::initBlocks() where piece_size is 0
this should not happen in production, but cover it anyway