Prevent test_pystache.py from expecting spec tests if spec test directory not present.
Prevent test_pystache.py from expecting spec tests if spec test directory not present.

Prior to this commit, running test_pystache.py from a downloaded sdist has
a test failure because the spec tests are not found.

rismalrv/pystache,jrnold/pystache,harsh00008/pystache,arlenesr28/pystache,charbeljc/pystache,beni55/pystache,nitish116/pystache,nitish116/pystache,nitish116/pystache,rismalrv/pystache,beni55/pystache,arlenesr28/pystache,charbeljc/pystache,rismalrv/pystache,arlenesr28/pystache,defunkt/pystache,harsh00008/pystache,jrnold/pystache,harsh00008/pystache