test_common: avoid needless fcntl in start_script

test_common: avoid needless fcntl in start_script

POSIX::dup2 does not do anything in addition to dup2(2) and is
thus immune to Perl automatically setting FD_CLOEXEC on FDs it
makes into IO objects/globs.  We only need to account for the
case when both args for dup2 are identical, in which case the
kernel treats it as a no-op and then thus we need to clear
FD_CLOEXEC ourselves.