Improve testing workflow

[fnikolai]

Description

Separate the testenv_image from testenv_init.
The first one is responbie for building the sandbox image with all its dependencies.
The second initializes the docker-compose, using as based the image of the previous step.

Related Issues

Checklist

• Is this code covered by new or existing unit tests or integration tests?
• Did you run make test successfully?
• Do new classes, functions, methods and parameters all have docstrings?
• Were existing docstrings updated, if necessary?
• Was external documentation updated, if necessary?

Additional Notes or Comments

[codecov-commenter]

Codecov Report

Attention: 102 lines in your changes are missing coverage. Please review.

Comparison is base (34830a7) 80.33% compared to head (1eca5a1) 80.39%.
Report is 77 commits behind head on main.

Files	Patch %	Lines
superduperdb/backends/ibis/query.py	74.21%	41 Missing ⚠️
superduperdb/backends/mongodb/query.py	83.33%	10 Missing ⚠️
superduperdb/cli/stack.py	0.00%	9 Missing ⚠️
superduperdb/base/datalayer.py	84.09%	7 Missing ⚠️
superduperdb/cdc/cdc.py	64.70%	6 Missing ⚠️
superduperdb/cli/serve.py	54.54%	5 Missing ⚠️
superduperdb/backends/sqlalchemy/metadata.py	73.33%	4 Missing ⚠️
superduperdb/base/build.py	71.42%	4 Missing ⚠️
superduperdb/backends/base/query.py	85.00%	3 Missing ⚠️
superduperdb/base/serializable.py	92.85%	3 Missing ⚠️
... and 6 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1347      +/-   ##
==========================================
+ Coverage   80.33%   80.39%   +0.05%     
==========================================
  Files          95      100       +5     
  Lines        6602     6887     +285     
==========================================
+ Hits         5304     5537     +233     
- Misses       1298     1350      +52     

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