Fix coverity complain in commandtest.c

Fix coverity complain in commandtest.c

For a "newfd1" the coverity tools thinks that the fd is closed in
a "virCommandPassFD", but with "flags == 0" it cannot be closed.

The code itself is ok, but coverity tool thinks that there is
"double_close" of the "newfd1" and to prevent showing this error
we simply add a comment before the proper close.

This has been found by coverity.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>