test: Run garbage collector before testing malloc numbers.

The gccgo library generates some garbage in the init routines
because it handles interfaces slightly differently.  Since the
test sets MemStats.Alloc to 0, the first time the garbage
collector runs it goes negative and the test fails.

R=rsc, r2
CC=golang-dev
https://golang.org/cl/2110044

test/mallocrep.go

@@ -31,6 +31,7 @@ func bigger() {
 }
 
 func main() {
+	runtime.GC()		   // clean up garbage from init
 	runtime.MemProfileRate = 0 // disable profiler
 	runtime.MemStats.Alloc = 0 // ignore stacks
 	flag.Parse()