test-filecache: use sys.executable to call python

As was mentioned in c102b704edb5, test scripts calling 'python' or 'python3'
might use the wrong python.
For test-filecache.py, this causes a failed test on CentOS 7.

Differential Revision: https://phab.mercurial-scm.org/D9295

tests/test-filecache.py

@@ -5,7 +5,7 b' import subprocess'
 import sys
 
 if subprocess.call(
-    ['python', '%s/hghave' % os.environ['TESTDIR'], 'cacheable']
+    [sys.executable, '%s/hghave' % os.environ['TESTDIR'], 'cacheable']
 ):
     sys.exit(80)