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

src/pkg/strconv/itoa_test.go

@@ -127,6 +127,7 @@ func TestUitoa(t *testing.T) {
 }
 
 func numAllocations(f func()) int {
+	runtime.GC()
 	memstats := new(runtime.MemStats)
 	runtime.ReadMemStats(memstats)
 	n0 := memstats.Mallocs