Fix Python and JavaScript (Karma) tests for Problem XBlock

[irtazaakram]

Summary

This PR fixes the Python and JavaScript (Karma) tests for the newly extracted Problem (CAPA) XBlock.

Context

• Extraction PR: Adds Problem XBlock extracting from edx-platform repo #63
• Platform Enablement PR: Test PR | Enable Extracted Problem XBlock openedx-platform#37136

What Changed

• Python Tests: Fixed import paths and module references in tests/ to align with the new directory structure.
• Javascript Tests: Added Karma configuration and Webpack config.
• Setup: Changed how xblocks-contribute is installed by requiring npm build.

Notes

• Pylint cleanup and documentation updates are out of scope for this PR and will be addressed in a follow-up contribution.
• The primary goal of this PR is to get the test suite green to unblock integration.