We use [mocha](https://mochajs.org/) as the test framework, as it is
the tool used by mathjs and we would like to make tests as similar
as possible. However, to tighten the linkage between source code and
tests, we adopt a somewhat different file organization: unit tests
for a given source file `blah/foo.js` are in `blah/__test__/foo.spec.js`.
To run all unit tests, execute the script `pnpm test`.
Resolves#3.
