Also: update fmt_test.go.

Together with the previous commits, we are now at or below c85b77c2
levels in terms of allocation for the benchmark described in #16897
(old = c85b77c2, new = this commit):

name       old time/op     new time/op     delta
Template       297ms ± 5%      284ms ± 3%  -4.53%  (p=0.000 n=27+29)
Unicode        159ms ± 5%      151ms ± 5%  -4.91%  (p=0.000 n=28+30)
GoTypes        985ms ± 5%      935ms ± 2%  -5.13%  (p=0.000 n=28+29)

name       old alloc/op    new alloc/op    delta
Template      46.8MB ± 0%     45.7MB ± 0%  -2.37%  (p=0.000 n=30+30)
Unicode       37.8MB ± 0%     37.9MB ± 0%  +0.29%  (p=0.000 n=29+30)
GoTypes        143MB ± 0%      138MB ± 0%  -3.64%  (p=0.000 n=29+30)

name       old allocs/op   new allocs/op   delta
Template        444k ± 0%       440k ± 0%  -0.94%  (p=0.000 n=30+30)
Unicode         369k ± 0%       369k ± 0%  +0.19%  (p=0.000 n=29+30)
GoTypes        1.35M ± 0%      1.34M ± 0%  -1.24%  (p=0.000 n=30+30)

For #16897.

Change-Id: Iedbeb408e2f1e68dd4a3201bf8813c8066ebf7ed
Reviewed-on: https://go-review.googlesource.com/29089Reviewed-by: Matthew Dempsky <mdempsky@google.com>