tests: avoid unlikely deadlock in dd/no-allocate on some shells

tests: avoid unlikely deadlock in dd/no-allocate on some shells

* test/dd/no-allocate.sh: Use 'wait' to ensure we don't have
multiple writers to the fifo, which was seen to trigger
a very hard to reproduce deadlock with make -j20 on solaris.
Also avoid writing to the fifo with the shell; instead using dd.
(check_dd_seek_alloc): A new function refactored from the various
cases, which are now constructed from function parameters.