[dev.garbage] runtime: Linear map test give false negative due to GC.

This test gives a false negative at an observed rate of 1 in a 1000
due to the fact that it runs for < 100 ms. allowing GC pauses to
warp the results. Changed the test so that it triggers only if it
remains non-linear for much larger problem sizes.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/164010043

test/maplinear.go

@@ -45,11 +45,11 @@ func checkLinear(typ string, tries int, f func(n int)) {
 			return
 		}
 		fails++
-		if fails == 6 {
+		if fails == 12 {
 			panic(fmt.Sprintf("%s: too slow: %d inserts: %v; %d inserts: %v\n",
 				typ, n, t1, 2*n, t2))
 		}
-		if fails < 4 {
+		if fails < 10 {
 			n *= 2
 		}
 	}