Create a test helper to test a reducer on fixtures
The idea was implemented in the BreadcrumbBar component:
Since this is going to be used in other tests, it is a good idea to move it into the test helpers:
Fixes #23468 - Add a test helper to test reducers with fixtures
The idea of testing a reducer using fixtures was introduced
in the BreadcrumBar tests.
Since, other components are going to be tested using this method,
it is moved to testHelpers.js to avoid duplication of code.
Signed-off-by: Boaz Shuster <firstname.lastname@example.org>