doc/go1.2.txt: testing: add TB interface

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/12890044

doc/go1.2.txt

@@ -65,6 +65,7 @@ strings: add IndexByte, for consistency with bytes package (CL 12214044).
 sync/atomic: add Swap functions (CL 12670045).
 syscall: implemented Sendfile for Darwin, added Syscall9 for Darwin/amd64 (CL 10980043).
 testing: AllocsPerRun is now quantized to an integer (the type is still float64) (CL 9837049).
+testing: add TB interface (intersection of T and B's methods) (CL 12962043).
 time: Allow Parse and Format to handle time zone offsets with seconds (CL 8132044)
 time: patterns require non-lowercase letter to follow Mon, Jan etc (CL 12448044).
 unicode: add In, a nicer-to-use but equivalent version of IsOneOf (CL 11672044).