Make stub QgisInterface and MockMessageBar inherit real interfaces

[Joonalai]

This PR:

• Makes stub implementations of QgisInterface and QgsMessageBar to inherit their real parents
• Improves tests

Fixes #24, fixes #26.

[codecov-commenter]

Codecov Report

Attention: 577 lines in your changes are missing coverage. Please review.

Comparison is base (af7f77a) 57.77% compared to head (64ad100) 29.85%.

Files	Patch %	Lines
src/pytest_qgis/qgis_interface.py	3.79%	558 Missing :warning:
src/pytest_qgis/mock_qgis_classes.py	58.13%	18 Missing :warning:
src/pytest_qgis/pytest_qgis.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #54 +/- ## =========================================== - Coverage 57.77% 29.85% -27.92% =========================================== Files 7 7 Lines 495 1065 +570 =========================================== + Hits 286 318 +32 - Misses 209 747 +538 ``` | [Flag](https://app.codecov.io/gh/GispoCoding/pytest-qgis/pull/54/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GispoCoding) | Coverage Δ | | |---|---|---| | [pytest](https://app.codecov.io/gh/GispoCoding/pytest-qgis/pull/54/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GispoCoding) | `29.85% <7.53%> (-27.92%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GispoCoding#carryforward-flags-in-the-pull-request-comment) to find out more.

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[Joonalai]

@LKajan, could you review this?