This is to confirm that `graph_diff` does not work with quads.

Other changes:
- Added support to `GraphHelper` for collapsing `BNode`s to a specific IRI
  when converting to triple sets, this is in addition to the current
  functionality for ignoring BNodes. This makes it possible to more
  accurately compare Graphs that contain blank nodes.
- Added `assert_cgraph_isomorphic` to `GraphHelper`.
  This method adds some rudimentary support for checking if conjunctive
  graphs are isomorphic, it will ignore named graphs that have blank
  nodes as name.

This patch converts some additional graph tests to pytest, I'm doing
this in preperation for adding more tests to `test_diff.py`.

This patch primarily adds more type hints for `rdflib.graph`, but also
adds type hints to some related modules in order to work with the new
type hints for `rdflib.graph`.

I'm mainly doing this as a baseline for adding type hints to
`rdflib.store`.

I have created type aliases to make it easier to type everything
consistently and to make type hints easier easier to change in the
future. The type aliases are private however (i.e. `_`-prefixed) and
should be kept as such for now.

This patch only contains typing changes and does not change runtime
behavior.

Broken off from https://github.com/RDFLib/rdflib/pull/1850

This patch moves all test utility modules into `test/utils/` and the
tests for these modules into `test/utils/test/`.

This is to make test utilities more organized and in preparation for
adding more test utilities related to test suites.

for more information, see https://pre-commit.ci