all: rename type *testing.B variable to 'b'

all: rename type *testing.B variable to 'b'

Reserve 't' for type *testing.T variables.

Change-Id: I037328df59d3af1aa28714f9efe15695b6fd62a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/400826
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>