The included binaries are patched to allow mining from scratch and difficulty as low as 0.00000381.
On Debian, to build for yourself:
PACK=bitcoin-0.8.5
git clone http://github.com/yebyen/testnet-debian.git; apt-get build-dep bitcoind; apt-get install devscripts
pushd testnet-debian
for i in *.tar.gz; do tar zxvf $i; done; mv $PACK/debian/{patches,changelog} .; rmdir $PACK/debian
mv patches/* debian/patches/; mv changelog debian/; mv debian $PACK/
pushd $PACK/ && dpkg-source --commit \
&& debuild -us -uc -b -rsudo; popd
cd $PACK/
rm src/leveldb/build_config.mk
rm -r src/obj-test/*
rm -r src/obj/*
rm -r build/
rm qrc_bitcoin.cpp
Then you simply install the bitcoind or bitcoin-qt deb that eventually is spit out, depending on your preference for graphical dependencies or not. Don't forget to wipe your blockchain and wallet if you are using testnet-in-a-box! You will need all of the QT libraries from build-dep even to compile bitcoind as a command line daemon only; they are quite interleaved in the build process.
The version 0.7.2-4 is set in the changelog file under $PACK/debian.