fix(web-server): cache static files

Files in `karma/static/*` were always read from file system.
These files typically don’t change (except when developing Karma itself)
and so it makes sense to cache them in memory.
Especially `./static/context.html`, which is read for every test run.

The main reason for this change is that when using native events
(non-polling), watching can easily hit the EMFILE limit and then this
`fs.readFile()` can be super delayed. This was easily reproducible
problem in AngularJS test suite.
chore: release v0.12.21