os/exec: deflake a test on Linux

Work around buggy(?) Linux /proc filesystem.

Fixes #7808

LGTM=iant
R=golang-codereviews, iant
CC=adg, golang-codereviews
https://golang.org/cl/90400044

src/pkg/os/exec/exec_test.go

@@ -224,11 +224,22 @@ func TestPipeLookPathLeak(t *testing.T) {
 			t.Fatal("unexpected success")
 		}
 	}
+	for triesLeft := 3; triesLeft >= 0; triesLeft-- {
 		open, lsof := numOpenFDS(t)
 		fdGrowth := open - fd0
 		if fdGrowth > 2 {
+			if triesLeft > 0 {
+				// Work around what appears to be a race with Linux's
+				// proc filesystem (as used by lsof). It seems to only
+				// be eventually consistent. Give it awhile to settle.
+				// See golang.org/issue/7808
+				time.Sleep(100 * time.Millisecond)
+				continue
+			}
 			t.Errorf("leaked %d fds; want ~0; have:\n%s\noriginally:\n%s", fdGrowth, lsof, lsof0)
 		}
+		break
+	}
 }
 
 func numOpenFDS(t *testing.T) (n int, lsof []byte) {