Add tests for root_redirect hook when logging in as coach

[jonboiser]

Summary

Following up on #7620 (comment), this

1. adds two tests to verify that facility and class coaches get redirected to the coach plugin when they login.
2. refactors the TestCase class and renames the individual test cases

Reviewer guidance

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References

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Contributor Checklist

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• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
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• If this is an important user-facing change, PR or related issue has a 'changelog' label
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Testing:

• Contributor has fully tested the PR manually
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Reviewer Checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[codecov]

Codecov Report

Merging #7621 into release-v0.14.x will decrease coverage by 0.00%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
kolibri/core/tasks/utils.py	87.03% <0.00%> (-5.56%)	⬇️