[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 wonder if we need similar checks like we have in ElasticsearchIntegrationTest if there is no transient / persistent cluster settings chance and fail if so...?