strconv: run garbage collection before counting allocations in test

strconv: run garbage collection before counting allocations in test

My theory is that the call to f() allocates, which triggers
a garbage collection, which itself may do some allocation,
which is being counted.  Running a garbage collection
before starting the test should avoid this problem.

Fixes #2894 (I hope).

R=golang-dev, bradfitz, nigeltao
CC=golang-dev
https://golang.org/cl/5685046