Test execution didn't catch a computation issue it ideally would have

https://github.com/googlefonts/fontc/issues/581#issuecomment-1825847869 notes:

> It's a bit concerning that the current test suite in `fontc/lib.rs`, which does contain a bunch of tests named `second_compile_*`, did not catch this issue. Only when fontc is run from the command line this panic can be reproduced. It appears as if the fontc tests `compile` method and the actual fontc run() function (that is used when invoking fontc main.rs as executable) do not do exactly the same thing..

Test execution deliberately single threads but I think it could still share more than it does now.