testing: allow concurrent use of T and B
testing: allow concurrent use of T and B

Notably, allow concurrent logging and failing.

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/6453045

sanjosh/sanjos100-tipc,d0f/go-zh,Triskite/willstone-goclone,scirelli/scirelli-go,webfd/go-zh,rflanagan/reginaldflanagan-project1,d0f/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,webfd/go-zh,mhennings/marcohennings-go,Triskite/willstone-goclone,Triskite/willstone-goclone,rdp/rogerpack2005-golang,d0f/go-zh,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,webfd/go-zh,scirelli/scirelli-go,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,d0f/go-zh,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,mhennings/marcohennings-go,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,Triskite/willstone-goclone,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,Triskite/willstone-goclone,bryanxu/go-zh,mhennings/marcohennings-go,scirelli/scirelli-go,Triskite/willstone-goclone,scirelli/scirelli-go,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,mhennings/marcohennings-go,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,webfd/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,d0f/go-zh,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,scirelli/scirelli-go,mhennings/marcohennings-go,bryanxu/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,d0f/go-zh,rdp/rogerpack2005-golang,scirelli/scirelli-go