testing: quantize AllocsPerRun

As the code now says:

We are forced to return a float64 because the API is silly, but do
the division as integers so we can ask if AllocsPerRun()==1
instead of AllocsPerRun()<2.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/9837049

src/pkg/testing/allocs.go

@@ -9,6 +9,7 @@ import (
 )
 
 // AllocsPerRun returns the average number of allocations during calls to f.
+// Although the return value has type float64, it will always be an integral value.
 //
 // To compute the number of allocations, the function will first be run once as
 // a warm-up.  The average number of allocations over the specified number of
@@ -36,6 +37,9 @@ func AllocsPerRun(runs int, f func()) (avg float64) {
 	runtime.ReadMemStats(&memstats)
 	mallocs += memstats.Mallocs
 
-	// Average the mallocs over the runs (not counting the warm-up)
-	return float64(mallocs) / float64(runs)
+	// Average the mallocs over the runs (not counting the warm-up).
+	// We are forced to return a float64 because the API is silly, but do
+	// the division as integers so we can ask if AllocsPerRun()==1
+	// instead of AllocsPerRun()<2.
+	return float64(mallocs / uint64(runs))
 }