This helps make the Zulip development environment somewhat more robust
to new contributors, since it will give them a nice warning if they
try running any of our development tools outside the Zulip virtualenv.
Fixes#3468.
Now, `tools/test-all` calls a new program called `tools/tests-tools`
that runs unit tests in `test_css_parser.py` and 'test_template_parser.py`.
This puts 100% line coverage on tools/lib/css_parser.py.
This puts about 50% line coverage on tools/lib/template_parser.py.