tests: replace some `python` by `"$PYTHON"` to not only use Python 2
I deleted my `python` binary as a test and a few tests started
failing.
Differential Revision: https://phab.mercurial-scm.org/D10250
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed