Allow explicit definition of "entry" assembly

[mnivet]

This PR is related to my comment in #403

I've not add this as a parser setting like I've suggest because it would have need more changes to make that non static. Furthermore the need can be seen as an advanced feature needed only in some unit tests.

This both reasons lead me to this design that allow consumer to setup AutoBuildMetadataAssembly in its production code, or only in its test code.