graphs/kruskal: add a test case to verify the correctness, fix styles (#2443)

* test/graphs/kruskal: adding a test case to verify the correctness of the algorithm

Fixes #2128

* grahps/kruskal: running psf/black

* graphs/kruskal: read edges in a friendlier fashion

Co-authored-by: Christian Clauss <cclauss@me.com>

* Update minimum_spanning_tree_kruskal.py

* fixup! Format Python code with psf/black push

* Update test_min_spanning_tree_kruskal.py

* updating DIRECTORY.md

Co-authored-by: Christian Clauss <cclauss@me.com>
Co-authored-by: John Law <johnlaw.po@gmail.com>
Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>