Python tests for DeviceAgentVector

[Robadob]

This is based on callback branch which exists in #997, as that PR fixes a bug that was blocking one of the tests.

Most appropriate tests have been ported from the C test suite, some have been removed where Python implementation of vector differs (e.g. begin()/insert() etc).
Closes #557

I also fixed a typographic issue in the C test suite. I haven't bothered to build/run that, as the fixed Python equivalent works.

Windows/Debug

(venv) C:\Users\Robadob\fgpu2\tests\swig\python>py.test pop/test_device_agent_vector.py
================================================= test session starts =================================================
platform win32 -- Python 3.9.12, pytest-7.2.0, pluggy-1.0.0
rootdir: C:\Users\Robadob\fgpu2\tests\swig\python
collected 21 items

pop\test_device_agent_vector.py .....................                                                            [100%]

================================================= 21 passed in 0.56s ==================================================

[ptheywood]

Looks good, but not tested.