The connectivity routine would consider symbols, overwriting unused
subgraphs for pins that were not in the change list. This is resolved
by updating the full connectivity to only use pins in the graph since
symbols are not connected independently.
In the process of adding QA tests for this change, additional issues
with the schematic QA were discovered. Specifically, we were not
properly setting the root sheet UUID. This was partially masked by a
const_cast setting of the RefDes in sch_symbol when called the RefDes
getter. This exposed the fact that our QA ERC numbers did not match the
schematic editor stand alone ERC numbers. So the test value for one
check needed to be updated
Fixes https://gitlab.com/kicad/code/kicad/-/issues/17528
* Fix issue handling symbols with multiple inheritance.
* Remove unused code from the symbol library manager object.
* Splits out the library buffer and symbol buffer object so the can be unit
tested without having to mock the symbol library manager object.
* Add unit tests for library buffer and symbol buffer objects.