Since a given dapp does not necessarily have a sense of what networks are supported within the wallet (and since some dapps rely exclusively on wallet_addEthereumChain) we should support the functionality of switching to a network when receiving a wallet_addEthereumChain message with a chainID that we already support.

The new check for whether a new custom network already exists changes
timing that was expected to be static. The relevant assertions are now
wrapped in retries that will wait a certain amount of time for the given
assertion to pass.

This fix is the result of adding a test to confirm a popup is not shown.
Notably, the behavior of switching the network silently requires extra
vetting; however, this commit implements it as that was the branch's
intent.