Force full sampling of memory allocations only for MemProf

Also reduce the number of allocations in this testcase, as it no
longer requires many allocations to trigger sampling.

src/runtime/testdata/testprog/memprof.go

@@ -14,8 +14,6 @@ import (
 )
 
 func init() {
-	// Force heap sampling for determinism.
-	runtime.MemProfileRate = 1
 	register("MemProf", MemProf)
 }
 
@@ -23,7 +21,10 @@ var memProfBuf bytes.Buffer
 var memProfStr string
 
 func MemProf() {
-	for i := 0; i < 1000; i++ {
+	// Force heap sampling for determinism.
+	runtime.MemProfileRate = 1
+
+	for i := 0; i < 10; i++ {
 		fmt.Fprintf(&memProfBuf, "%*d\n", i, i)
 	}
 	memProfStr = memProfBuf.String()