unix: skip TestPselect on EINTR

Some slow builders occassionally seem to fail with:

    syscall_linux_test.go:301: Pselect: interrupted system call

Just skip the test in these cases for now.

Fixes golang/go#35555

Change-Id: I8850634fc4000bf9d0d6cce27834febc53d3fcd6
Reviewed-on: https://go-review.googlesource.com/c/sys/+/206862
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_linux_test.go

@@ -297,7 +297,9 @@ func TestPselect(t *testing.T) {
 	start := time.Now()
 	_, err = unix.Pselect(0, nil, nil, nil, &ts, nil)
 	took := time.Since(start)
-	if err != nil {
+	if err == unix.EINTR {
+		t.Skipf("Pselect interrupted after %v timeout", took)
+	} else if err != nil {
 		t.Fatalf("Pselect: %v", err)
 	}