Change the way that the `FILTER` option works in `make test FILTER=<pattern>`
(or equivalently the `--grep` option if using `gulp test --grep
<pattern>`) to control which test files are executed rather than
matching test descriptions.

This enables the test configuration function in `src/karma.config.js` to
limit the test bundle to only the matching files and their dependencies.
This can make the initial bundling much faster and bundle updates (when
using `make servetests`) somewhat faster.

On my system, `make test FILTER=menu-item` takes ~40s on master but only
~7s on this branch.

As before, the filename filter can be combined with the `.only` modifier
on `description` or `it` calls to do more fine-grained filtering of
tests within matching files.

 - Change `--grep` option to gulp to filter test files as part of the
   test bundling process instead of setting the `grep` mocha option

 - Make `make servetests` accept a `FILTER` argument for consistency
   with `make test`

 - Simplify code in gulpfile.js for launching Karma

 - Update documentation for `make test`