TB2::SyncStore::File->_opened_by_pid should not have a default.

The default would be a lie, since it should only be set at the point
of open.  Default it to 0 to avoid uninitialized value warnings when
checking it against $$.

Thanks @dakkar for noticing this.
Fix use Test::More "no_plan", coordinate_forks => 1

Extend the "no_plan" normalization to cover a list of arguments instead
of just one.