ERP5Catalog.test: Move indexation-content-checking tests to a separate file

To not receive catalog already tainted by clearing & reindexing.

In turn, this allows testing catalog reference content for equality:
catalog must be exactly the same before/after an ERP5Site_reindexAll, and
before/after a hot-reindex. So issubset is not an acceptable method.
Instead, use assertItemsEqual, and raise max diff size.

Also, tidy code a bit:
- Avoid clearing catalog only to restrict result set.
- Improve coding style.
- Simplify code (single-use variables...).
- Call assert methods producing more helpful error messages on failure.