fix: issue running tests when mainnet is default network and specifying a different network

[antazoey]

What I did

fixes: #2342

How I did it

Move the config error to a later point, check for the --network flag.

How to verify it

cfg:

ethereum:
  default_network: mainnet
  mainnet:
    default_provider: node
  mainnet_fork:
    default_provider: foundry

then run

ape test

should fail

and run

ape test --network ethereum:local:test

should not fail

Checklist

• All changes are completed
• New test cases have been added
• Documentation has been updated