Code redundancy between ext implementation and testing. (#3830)

* fix child_storage_hash

* extract common implementation for ext and testing

* cleaning impl.

* replace ExtBasisMut by actual Ext

* remove extbasis.

* Update tests to use Ext from test externalities.

* use Ext constructor for getting ext from TestExternalities.

* Add missing extensions from ext.

* fix wasmi test

* Fix merge error.