Remove unnecessary objects from VPI discovery

[ktbarrett]

Closes #3353.

Need investigation into whether the following should also be removed.

• vpiGate
• vpiPort
• vpiPrimTerm
• vpiRefObj

[ktbarrett]

Perhaps we need to add a case where we expect the type, but throw the object away because we know it isn't valuable?

[ktbarrett]

Apparently there is a test to see if we can find a SV gate, but I'm not sure yet finding gates is even useful, so I'm testing to see if the input and output "ports" can be found.

[ktbarrett]

We can't see any signals under a gate object, so their discovery is not important. The access_gate test will be removed.

[ktbarrett]

This is actually a bigger PITA than expected. Our "expected don't cares" are still passed back up to the GPI layer as NULLs, which get encoded as GPI_NOT_NATIVE objects, causing an error. This occurs when iterators over hierarchies are done.

[ktbarrett]

The discovery tests are now not finding certain objects. Investigation is needed to find out what's now missing after removing objects from iteration.

[ktbarrett]

Rerunning CI.