If the initial network when the extension is started is something other
than Mainnet, the swaps controller will never successfully retrieve
swap quotes. This is because the `ethers` provider used by the swaps
controller doesn't allow network changes by default - it assumes that
the network remains the same as when the provider was initialized.
This was fixed by hard-coding Mainnet as the initial chain ID for this
`ethers` provider used by the swaps controller.
Some adjustments needed to be made to the `provider` stub to allow
setting `1` as the network ID and chain ID in unit tests.
