all: protect alloc count tests by -testing.short

Update issue 5000
Should reduce the flakiness a little. Malloc counting is important
to general testing but not to the build dashboard, which uses -short.

R=golang-dev, dsymonds
CC=golang-dev
https://codereview.appspot.com/12866047
C: add Todd Wang (Google CLA).

R=r, adg
CC=golang-dev, toddwang
https://codereview.appspot.com/13149043