Validation to check that outcome and treatment aren't the same

[jmafoster1]

Closes #114. Added validation in BaseTestCase to check that treatment and outcome are not the same.

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ PYTHON	black	32		0	1.02s
✅ PYTHON	pylint	32		0	7.14s

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.75%. Comparing base (87f07ec) to head (f2392dd).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #336   +/-   ##
=======================================
  Coverage   95.75%   95.75%           
=======================================
  Files          27       27           
  Lines        1627     1627           
=======================================
  Hits         1558     1558           
  Misses         69       69           

Files with missing lines	Coverage Δ
causal_testing/testing/base_test_case.py	100.00% <100.00%> (ø)

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