Test feature parity between PSQL and SQLite DB backends

[GeigerJ2]

Currently, there are various tests that are marked with requires_psql, e.g., here:

aiida-core/tests/orm/test_querybuilder.py

Lines 727 to 732 in 779cc29

	@pytest.mark.requires_psql
	def test_as_sql(self):
	"""Test ``qb.as_sql(inline=False)`` returns the correct string."""
	qb = orm.QueryBuilder()
	qb.append(orm.Node, project=['uuid'], filters={'extras.tag4': 'appl_pecoal'})
	self.regress_str(qb.as_sql(inline=False))

We should check which of those tests are missing for SQLite (such as def test_as_sql linked above that is, due to the mark, only ever run for PSQL), which ones should actually work for both, and which ones truly require PSQL (e.g., PSQL-backend specific ones, of course), to achieve testing feature parity between the different SQL backends.

Pinging @danielhollas and @rabbull.