testcommon: fix run_script for older Perls

Using Perl "open" to dup(2) and save the old handles is required
since "local *STDIN = *STDIN" does not work on old Perls.  Even
worse, this was silently a no-op when tested with Perl 5.24.1 on
Debian 9.x and led to confusing failures in the t/httpd-corner.t
lsof(1) tests when run after t/v2mirror.t from the same worker
process using t/run.perl.

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