Update testing routine with fixtures and benchmark notebooks test

[martinvonk]

Short Description

1. Move pytest fixture to conftest.py
   Fixutres are autodetected by pytest in conftest.py and don't have to be imported anymore.
   For more info: https://docs.pytest.org/en/6.2.x/fixture.html

2. Test benchmark notebooks on pull request to master branch
   Fixes point 3 of [ENHANCEMENT] Update (unit) testing and GitHub continuous integration with tox #509

[codacy-production]

Coverage summary from Codacy

Merging #583 (5c74d9c) into dev (7a9e44c) - See PR on Codacy

Coverage variation	Diff coverage
✅ +0.00% (target: +0.00%)	100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (7a9e44c)	4914	3569	72.63%
Head commit (5c74d9c)	4914 (+0)	3569 (+0)	72.63% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#583)	1	1	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[martinvonk]

Maybe we can merge this @dbrakenhoff ?

[dbrakenhoff]

The blocked check is preventing me from merging. I've removed the test suite for Jupyter Notebooks from required status checks.

It allows me to merge this PR, but we need to check if we need to re-add that status check.