fix(core): Reorganize core tests and improve

[Tranquility2]

Found some loss ends while working on fixing the typing.

The idea in this PR is to reorganize the tests to better reflect the use case (not all just in test_core)
This should help help avoid later issues when we fix more typing (allowing better isolation/management)
Also took this opportunity to add missing tests for utils and config

So now we have:

1. test_image - stripped from test_core
2. test_waiting_utils - stripped from test_core
3. a much cleaner test_core
4. test_utils - new
5. test_config - new

---------- coverage: platform linux, python 3.10.12-final-0 ----------
Name                                        Stmts   Miss Branch BrPart  Cover   Missing
---------------------------------------------------------------------------------------
core/testcontainers/core/__init__.py            0      0      0      0   100%
core/testcontainers/core/auth.py               31      0     12      0   100%
core/testcontainers/core/config.py             59      0     20      0   100%
core/testcontainers/core/container.py         159     23     40      9    80%   20, 61-62, 82-84, 121->123, 135, 138, 158-162, 186, 191, 244->260, 250-259, 261
core/testcontainers/core/docker_client.py     118     18     32      9    79%   63-65, 96, 134->133, 138-139, 141, 152, 161, 179, 196, 200-201, 204-208
core/testcontainers/core/exceptions.py          3      0      0      0   100%
core/testcontainers/core/generic.py            28     28      0      0     0%   13-82
core/testcontainers/core/image.py              47      2      6      2    92%   10, 68
core/testcontainers/core/labels.py             23      0      8      0   100%
core/testcontainers/core/network.py            20      1      0      0    95%   30
core/testcontainers/core/utils.py              46      9     10      2    77%   27->exit, 63-70, 79
core/testcontainers/core/version.py            20      0     12      0   100%
core/testcontainers/core/waiting_utils.py      48      8     14      4    81%   26, 60-67, 77, 104->106, 122
---------------------------------------------------------------------------------------
TOTAL                                         602     89    154     26    83%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #693   +/-   ##
=======================================
  Coverage        ?   81.56%           
=======================================
  Files           ?       12           
  Lines           ?      602           
  Branches        ?       89           
=======================================
  Hits            ?      491           
  Misses          ?       85           
  Partials        ?       26           

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