Move test fixture and helpers to internal package

Make TestContext our public API for unit tests. Some of the helpers
might also be of interest for application tests, we'll revise and
publish them when needed.

Change-Id: I7445d97d81e5c633cdc74c3ad71e0eecb6a986f8
Signed-off-by: Ralf Sternberg <rsternberg@eclipsesource.com>