Provide helper functions to test contract upgrades and migrations

[facuspagnuolo]

Provide helper functions so any project that needs to perform a contract upgrade can actually test that a certain upgrade is working as expected.

Some things we should consider:

• The different versions of the same smart contracts may not be available
• Users shouldn't be aware of how to deal with proxies
• We may want to provide some assertion helpers

[spalladino]

Depends on #1309