When considering a sequence's Data entry in dumpSequenceData, we were
actually looking at the sequence definition's dump flag to decide if we
should dump the data or not.  That's generally fine, except for when the
sequence data entry was created by processExtensionTables() because it's
a config sequence.  In that case, the sequence itself won't be marked as
dumping data because it's part of an extension, leading to the need for
processExtensionTables() to create the sequence data entry.

This leads to extension config sequence data not being included in the
dump when it should be.  Fix this by looking at the sequence data's dump
flag instead, just as dumpTableData() was doing for tables (which is why
config tables were correctly being handled), and add a regression test
to make sure we don't break it moving forward.

All of this is a bit round-about since we can now represent which
components of a given dump item should be dumped out through the dump
flag.  A future improvement might be to change checkExtensionMembership()
to check for config sequences/tables and set the dump flag based on that
directly, possibly removing the need for processExtensionTables().

Bug found by Daniele Varrazzo.

Discussion: https://postgr.es/m/CA+mi_8ZmxQM7+nZ7pJ8uyfxc9V3o=UAG14dVqvftdmvw8OJ3gQ@mail.gmail.com

Patch by Michael Paquier, with some tweaking of the regression tests by
me.

Back-patch to 9.6 where the bug was introduced.