complete-run: close all fds except for 0, 1, 2

running in a VIM subshell leads to one more fd, for example

testcases/lib/SocketActivation.pm

@@ -7,6 +7,7 @@ use IO::Socket::UNIX; # core
 use Cwd qw(abs_path); # core
 use POSIX qw(:fcntl_h); # core
 use AnyEvent::Handle; # not core
+use AnyEvent::Util; # not core
 use Exporter 'import';
 use v5.10;
 
@@ -78,6 +79,10 @@ sub activate_i3 {
             POSIX::close(fileno($socket));
         }
 
+        # Make sure no file descriptors are open. Strangely, I got an open file
+        # descriptor pointing to AnyEvent/Impl/EV.pm when testing.
+        AnyEvent::Util::close_all_fds_except(0, 1, 2, 3);
+
         # Construct the command to launch i3. Use maximum debug level, disable
         # the interactive signalhandler to make it crash immediately instead.
         my $i3cmd = abs_path("../i3") . " -V -d all --disable-signalhandler";