runtime: fix typos in test

I don't know what is n, but it exists somewhere there.

LGTM=dave
R=golang-codereviews, dave
CC=golang-codereviews
https://golang.org/cl/58710043

src/pkg/runtime/chan_test.go

@@ -136,10 +136,10 @@ func TestChan(t *testing.T) {
 			for i := 0; i < 100; i++ {
 				v, ok := <-c
 				if !ok {
-					t.Fatalf("chan[%d]: receive failed, expected %v", n, i)
+					t.Fatalf("chan[%d]: receive failed, expected %v", chanCap, i)
 				}
 				if v != i {
-					t.Fatalf("chan[%d]: received %v, expected %v", n, v, i)
+					t.Fatalf("chan[%d]: received %v, expected %v", chanCap, v, i)
 				}
 			}
 
@@ -172,11 +172,11 @@ func TestChan(t *testing.T) {
 				}
 			}
 			if len(recv) != L {
-				t.Fatalf("chan[%d]: received %v values, expected %v", n, len(recv), L)
+				t.Fatalf("chan[%d]: received %v values, expected %v", chanCap, len(recv), L)
 			}
 			for _, v := range recv {
 				if v != P {
-					t.Fatalf("chan[%d]: received %v values, expected %v", n, v, P)
+					t.Fatalf("chan[%d]: received %v values, expected %v", chanCap, v, P)
 				}
 			}
 		}