This commit removes the final callbacks in this code by switching to a
promises-based interface, overall simplifying the code. Moreover, we
document why we write to files on disk and modernize the code using e.g.
template strings.
The original `test.py` code, see
c2376e5cea/test/test.py,
did not have any timeout logic for TTX, but it got introduced when
`test.py` was ported from Python to JavaScript as `test.js` in
c2376e5cea (diff-a561630bb56b82342bc66697aee2ad96efddcbc9d150665abd6fb7ecb7c0ab2f).
However, I don't think we've ever actually seen TTX timing out.
Moreover, back then we used a very old version of TTX and ran the font
tests on the bots (where a hanging process would block other jobs and
would require a manual action to fix), so this code was most likely
only included defensively.
Fortunately, nowadays it should not be necessary anymore because we use
the most recent version of TTX (which either returns the result or
errors out, but isn't known to hang on inputs) and we run the font tests
on GitHub Actions which doesn't block other jobs anymore and also
automatically times the job out for us in the unlikely event that a hang
would ever occur.
In short, we can safely remove this logic to simplify the code and to get
rid of a callback.
This commit migrates the font tests away from the bots. Not only are the
font tests broken on the Windows bot since some time, they also run on
Python 2 (end of life since January 2020) and `ttx` 3.19.0 (released in
November 2017). The latter is installed via a submodule, which requires
more complicated logic for finding and running `ttx`.
We solve the issues by implementing a modern workflow that installs the
most recent stable Python and `ttx` (`fonttools` package) versions. This
simplifies the `ttx` driver code as well because it can now assume `ttx`
is available on the path (just like we do for e.g. `node` invocations).
GitHub Actions takes care of creating a virtual environment with
`fonttools` in it so that the `ttx` entrypoint is available. Locally
the font tests can be run in a similar way by creating and sourcing a
virtual environment with `fonttools` in it before running the font
tests, and a README file is included with instructions for doing so.
