test-filecache: use sys.executable to call python...
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
