tests: ignore pidfd in fd leak test

This is not a real leak:

  fd leak in test process? before, after:
  [0r=/dev/null 3r=/proc/940141/fd 5rw=anon_inode:[eventfd] (filtered: pipe:[2454797], pipe:[2454797], anon_inode:[eventpoll])]
  [0r=/dev/null 3r=/proc/940141/fd 5rw=anon_inode:[eventfd] 12rw=anon_inode:[pidfd] (filtered: pipe:[2454797], pipe:[2454797], anon_inode:[eventpoll], pipe:[2460158])]

Ignore pidfd.

tests/test_helpers/mount_unmount.go

@@ -242,11 +242,12 @@ func ListFds(pid int, prefix string) []string {
 			// fd was closed in the meantime
 			continue
 		}
-		if strings.HasPrefix(target, "pipe:") || strings.HasPrefix(target, "anon_inode:[eventpoll]") {
+		if strings.HasPrefix(target, "pipe:") || strings.HasPrefix(target, "anon_inode:[eventpoll]") ||
+			strings.HasPrefix(target, "anon_inode:[pidfd]") {
 			// The Go runtime creates pipes on demand for splice(), which
 			// creates spurious test failures. Ignore all pipes.
-			// Also get rid of the "eventpoll" fd that is always there and not
-			// interesting.
+			// Also get rid of the "eventpoll" and "pidfd" fds that are always there
+			// and not interesting.
 			filtered = append(filtered, target)
 			continue
 		}