Add integration test for backend-specific imports

[nkovela1]

This PR adds an integration test to ensure that backend-specific imports are not used.

It works by creating a virtual environment where only one backend's packages are installed and then testing a basic subclassed model.fit() flow.

We can integrate this with CI in the next PR or this one.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -11.55% ⚠️

Comparison is base (180e134) 83.82% compared to head (347b3d7) 72.28%.
Report is 15 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #945       +/-   ##
===========================================
- Coverage   83.82%   72.28%   -11.55%     
===========================================
  Files         318      319        +1     
  Lines       28737    28879      +142     
  Branches     5489     5529       +40     
===========================================
- Hits        24090    20874     -3216     
- Misses       3141     6620     +3479     
+ Partials     1506     1385      -121     

Flag	Coverage Δ
keras_core	72.22% <ø> (-11.50%)	⬇️
keras_core-jax	?
keras_core-numpy	60.56% <ø> (-0.08%)	⬇️
keras_core-tensorflow	67.01% <ø> (+0.04%)	⬆️
keras_core-torch	?

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see 70 files with indirect coverage changes

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[fchollet]

Thanks for the PR! Please reopen it in keras-team/keras.