Move test_hooks.py code

[carmocca]

What does this PR do?

Move HookedModel class out of the test
Split test into 3, {fit,validate,test}

Reducing the diff for #7682

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[codecov]

Codecov Report

Merging #7689 (f2a8ce9) into master (8b01497) will decrease coverage by 49%.
The diff coverage is n/a.

❗ Current head f2a8ce9 differs from pull request most recent head 67d15c2. Consider uploading reports for the commit 67d15c2 to get more accurate results

@@           Coverage Diff            @@
##           master   #7689     +/-   ##
========================================
- Coverage      93%     44%    -49%     
========================================
  Files         199     199             
  Lines       12957   12954      -3     
========================================
- Hits        11991    5691   -6300     
- Misses        966    7263   +6297