Fixed "np.int64" is not JSON encodable from test outcomes

[jmafoster1]

Small fix to the Positive and Negative test outcomes such that they return native bool datatypes rather than np.bool, resulting in np datatypes being carried forward into other parts of the CTF and causing JSON encoding issues further down the line.

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ PYTHON	black	26		0	1.49s
✅ PYTHON	pylint	26		0	5.65s

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[codecov]

Codecov Report

Merging #241 (189c210) into main (e64469e) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #241   +/-   ##
=======================================
  Coverage   95.45%   95.45%           
=======================================
  Files          20       20           
  Lines        1408     1408           
=======================================
  Hits         1344     1344           
  Misses         64       64           

Files Changed	Coverage Δ
causal_testing/testing/causal_test_outcome.py	96.96% <100.00%> (ø)

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[f-allian]

Approving changes