os/exec: skip possible netpoll pipe in known FDs in test

Fixes #35045

Change-Id: I90ac29882c7d03936c98c4116a8bccdd2ecbf76b
Reviewed-on: https://go-review.googlesource.com/c/go/+/202445
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

src/os/exec/exec_test.go

@@ -453,13 +453,13 @@ var testedAlreadyLeaked = false
 
 // basefds returns the number of expected file descriptors
 // to be present in a process at start.
-// stdin, stdout, stderr, epoll/kqueue, maybe testlog
+// stdin, stdout, stderr, epoll/kqueue, epoll/kqueue pipe, maybe testlog
 func basefds() uintptr {
 	n := os.Stderr.Fd() + 1
 	// The poll (epoll/kqueue) descriptor can be numerically
 	// either between stderr and the testlog-fd, or after
 	// testlog-fd.
-	if poll.IsPollDescriptor(n) {
+	for poll.IsPollDescriptor(n) {
 		n++
 	}
 	for _, arg := range os.Args {