tests import path: put shipped pmb code first

Always test the pmb code from the parent folder of the tests, even if
a stable version of pmbootstrap is installed with pip or pysetup.

test/test_frontend.py

@@ -21,7 +21,7 @@ import sys
 import pytest
 
 # Import from parent directory
-sys.path.append(os.path.realpath(
+sys.path.insert(0, os.path.realpath(
     os.path.join(os.path.dirname(__file__) + "/..")))
 import pmb.config
 import pmb.parse