[TESTS] Add ElasticsearchSingleNodeTest.

This test makes it easy to create a lightweight node (no http, indices stored
in RAM, ...) whose main purpose is to get an instance of the Guice injector
for unit tests.

This should help not have to update lots of unit tests when we add a new
Guice dependency.
I think we should try to make the impact of this even lower and make all the threadpools `scaling` with `0s` `keep_alive`?