Add more representative unit tests for load_experiment(skip_runners_and_metrics=True)

[mgrange1998]

Summary:
Adds testing for skip_runners_and_metrics for with and without reduced_state being specified. T219831930

Adds three tests below

1. test_load_without_skip_raises_json_decode_error
2. test_load_experiment_with_aux_exp_and_custom_metric_in_gen_metadata
3. test_resave_experiment_with_aux_exp_loses_custom_metrics_and_runner

JSON Decoding Error

ApplicationError: (<ApplicationErrorType.UNKNOWN: 0>, 'JSONDecodeError("The JSON 
dictionary passed to `object_from_json` has a type Deltoid3Metric that is not registered with a corresponding class in DECODER_REGISTRY. 
Please see our storage tutorial (https://ax.dev/docs/storage.html) for more details (\'Customizing\' section will be relevant for saving Ax object subclasses).")')

Differential Revision: D91349882

[meta-codesync]

@mgrange1998 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D91349882.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.72%. Comparing base (0a85ca1) to head (cd06c62).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4817   +/-   ##
=======================================
  Coverage   96.71%   96.72%           
=======================================
  Files         586      587    +1     
  Lines       61307    61385   +78     
=======================================
+ Hits        59296    59373   +77     
- Misses       2011     2012    +1     

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