test-check-interface: do not expect bare "python" executable exists

The test would crash if python executable had version suffix.

tests/test-check-interfaces.py

@@ -12,7 +12,7 b' import sys'
 
 # Only run if tests are run in a repo
 if subprocess.call(
-    ['python', '%s/hghave' % os.environ['TESTDIR'], 'test-repo']
+    [sys.executable, '%s/hghave' % os.environ['TESTDIR'], 'test-repo']
 ):
     sys.exit(80)