Refactor fixtures

[blink1073]

Move more logic and fixtures including io_loop into core so they can be shared.
Prefix fixture names where appropriate.
Automatically install echo kernel in base environment.

[codecov]

Codecov Report

Base: 94.82% // Head: 94.39% // Decreases project coverage by -0.43% ⚠️

Coverage data is based on head (e364a1a) compared to base (8b054c0).
Patch coverage: 95.16% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
- Coverage   94.82%   94.39%   -0.44%     
==========================================
  Files          10       10              
  Lines         464      464              
==========================================
- Hits          440      438       -2     
- Misses         24       26       +2     

Impacted Files	Coverage Δ
pytest_jupyter/__init__.py	100.00% <ø> (ø)
pytest_jupyter/jupyter_core.py	96.42% <93.18%> (-3.58%)	⬇️
pytest_jupyter/jupyter_client.py	89.65% <100.00%> (-3.45%)	⬇️
pytest_jupyter/jupyter_server.py	92.69% <100.00%> (-0.30%)	⬇️
tests/test_jupyter_client.py	100.00% <100.00%> (ø)

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