Revert "os: remove skipped test on netbsd"

This reverts commit a631dab.

Reason for revert: I was wrong. It still fails on the builders.

I don't know what's different about my VMWare VM, but on GCE it fails.

Change-Id: Ic6bee494b69235768bf08ba0bf59026bca41ad12
Reviewed-on: https://go-review.googlesource.com/80915
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/os_test.go

@@ -1189,6 +1189,8 @@ func testChtimes(t *testing.T, name string) {
 			// content.  Similarly, atime is set whenever
 			// the contents are accessed; also, it is set
 			// whenever mtime is set.
+		case "netbsd":
+			t.Logf("AccessTime didn't go backwards; was=%v, after=%v (Ignoring. See NetBSD issue golang.org/issue/19293)", at, pat)
 		default:
 			t.Errorf("AccessTime didn't go backwards; was=%v, after=%v", at, pat)
 		}