Move test-read-dwarf.cc to abigail::workers

Move test-read-dwarf.cc to abigail::workers

Moving this test away from using pthreads directly to use
abigail::workers.

This patch also updates the valgrind suppression file to suppress some
Helgrind false positives.  Those are due to:
  - libstdc++ apparently having some benign data races when emitting
  data to ostream.  This seems related to some facet manipulation that
  happen at that point.
  - some benign data race in some elfutils functions.

	* tests/test-read-dwarf.cc (iospec, spec_lock, write_lock)
	(out_abi_base, in_elf_base, in_abi_base): Remove these global
	variables.
	(handle_in_out_spec): Remove this.
	(struct test_task): Write this task that does what
	handle_in_out_spec was doing.
	(test_task_sptr): Define new typedef.
	(main): Remove the pthreads artifacts.  Use the new test_task type
	along with the abigail::workers interface.
	* tests/test-valgrind-suppressions.supp: Add more helgrind
	suppressions for ostream writting false positives.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>