[Pkg-javascript-devel] Note for jest

[Xavier]

I added a debian/README.Debian (next release). Here is the content
(comments/English-fixes welcome):

# USING JEST IN DEBIAN PACKAGING

Changes added to jest package in default configuration:
 * jest package excludes <rootdir>.pc/ directory to avoid problems when
   upstream tests are patched
   (default value of --modulePathIgnorePatterns)
 * jest package includes Debian nodejs directories
   (default values of --modulePaths)

Usage notes:
 * when upstream installed snapshots in its test files, it may generate
   false-positive autopkgtest regressions (test succeeds but output
   changed). To avoid this, use the "-u" option: this updates snapshots
   and so avoid these kind of errors
 * always use the "--ci" option when jest is used in Debian packaging
   and/or autopkgtest. It prevents snapshot from being written
 * when your package contains component, it is recommended to set a path
   to jest to avoid launching component test that may be incompatible
   (component tests can be launched using
   debian/nodejs/<component>/test). Example:

     $ jest test/

 * pkg-js-autopkgtest
   * jest needs often babel configuration file. Since version 0.9.48,
     pkg-js-autopkgtest automatically installs .babelrc and
     babel.config.js in test directory (and installed package.json. If
     your package uses another file, set a debian/tests/pkg-js/files
     (see below)
   * jest tests often need sources files. Use debian/tests/pkg-js/files
     in this case.
   * debian/tests/pkg-js/files example (remember that when this file
     exists, only the mentionned files and package files are installed
     in test directory):

       .babelrc
       src/
       test/