Refactor old test cases and write some more test cases for views

[sumit4613]

Hey, I just made a Pull Request!

Description

Motivation and Context

Have you tested this? If so, how?

Checklist for PR author(s)

• Changes are covered by unit tests (no major decrease in code coverage %).
• All tests pass.
• Docstring coverage is 100% via interrogate drf_user (I mean, we should set a good example 😄).
• Updates to documentation:
  • Document any relevant additions/changes in README.md.
  • Manually update both the README.md and docs/index.rst for any new changes.
  • Any changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives. Find the appropriate next version in the project's __init__.py file.

Release note

[codecov]

Codecov Report

Merging #33 into master will increase coverage by 8.62%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
+ Coverage   49.08%   57.71%   +8.62%     
==========================================
  Files          17       17              
  Lines         601      603       +2     
==========================================
+ Hits          295      348      +53     
+ Misses        306      255      -51     

Flag	Coverage Δ
#unittests	57.71% <100.00%> (+8.62%)	⬆️

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Impacted Files	Coverage Δ
drf_user/serializers.py	53.00% <100.00%> (+14.45%)	⬆️
drf_user/views.py	57.76% <100.00%> (+3.72%)	⬆️
drf_user/utils.py	22.98% <0.00%> (+7.25%)	⬆️
drf_user/managers.py	62.06% <0.00%> (+41.37%)	⬆️
__init__.py	77.41% <0.00%> (+41.93%)	⬆️

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