Refactored unit tests for Astar and DP

[ignavierng]

Updates

• Refactored unit tests for Astar and DP into a single function for exact search

Test Plan

python -m unittest tests.TestExactSearch # should pass

[tofuwen]

Thanks, great work!
Btw, you will also change other tests to remove the sys.path.append(), right?

cc @kunwuz to merge this PR.

[ignavierng]

Yes, I will change the other tests to update the data path and remove sys.path.append(), so that in the future we should perhaps run python -m unittest tests.FileName, rather than cd-ing into tests folder and running python -m unittest FileName, the latter which imports causallearn (if exists) from pip installed packages instead of local one.