testing: add TB, an interface common to T and B

R=golang-dev, kevlar, rsc, adg, r
CC=golang-dev
https://codereview.appspot.com/12962043
archive/zip: speed up Zip64 test

Took 76 seconds or so before. By avoiding flate and crc32 on
4GB of data, it's now only 12 seconds.  Still a slow test, but
not painful to run anymore when you forget -short.

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/12950043