Fix 'make check' for python tests

[hamarituc]

Your checklist for this pull request

• not applicable --> I've documented or updated the documentation of every API function and struct this PR changes.
• not applicable --> I've added tests that prove my fix is effective or that my feature works (if possible)

Detailed description

The python tests were relocated to a subdirectory, but the check target in the Makefile wasn't changed accordingly. This PR changes the Makefile to execute the checks from the subdirectory.

Test plan

• Build Capstone with Python bindings
• Run make check

Closing issues

N/A