Update end-to-end tests for the graph-based decomposition

[maliasadi]

Context:
Added a new test file frontend/test/pytest/from_plxpr/test_decompose_transform.py just for the graph-based decomposition.

Description of the Change:

• Removed decompose-lowering tests from test_from_plxpr.py
• Added decompose-lowering tests to test_decompose_transform.py

Benefits:

• Better scoping test_from_plxpr.py
• Dedicated test file for decompose-lowering

Possible Drawbacks:

Related GitHub Issues:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.49%. Comparing base (526b58b) to head (5158532).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2221   +/-   ##
=======================================
  Coverage   97.49%   97.49%           
=======================================
  Files          93       93           
  Lines       10890    10890           
  Branches     1038     1038           
=======================================
  Hits        10617    10617           
  Misses        211      211           
  Partials       62       62           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[rniczh]

LGTM 👍

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md on your branch with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[paul0403]

💯