Refactor ReadTreeTest to allow testing other checkout classes

ReadTreeTest contains a lot of useful tests for "checkout"
implementations. But ReadTreeTest was hardcoded to test only
WorkDirCheckout. This change doesn't add/modify any tests semantically
but refactors ReadTreeTest so that a different implementations of
checkout can be tested. This was done to allow DirCacheCheckout to be
tested without rewriting all these tests.

Change-Id: I36e34264482b855ed22c9dde98824f573cf8ae22
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>