Validate that pghstore._native.dump works as expected

[sigmavirus24]

I believe we test this, but we also have a dump function defined insrc/pghstore/__init__.py with an incompatible signature for the case when _speedups is in play. Both need to have matching signatures and we need to test that both work as expected in either case with those additional parameters.

Warning This may require delving into _speedups.c to introduce similar behaviour to a dump function there.