refactor(tests): refactor redundant installation tests with network calls

We have 3 tests that actually attempt to install a toolchain in [tests/toolchain.rs](https://github.com/FuelLabs/fuelup/blob/92025fa33607010c40868bb971cb0e2d0119a193/tests/toolchain.rs):

- fuelup_toolchain_install_latest
- fuelup_toolchain_install_nightly
- fuelup_toolchain_install_nightly_date

We could probably do with just 1 of these just as a sanity check, say `latest`, but the rest could probably be refactored as input validation tests instead. That is, ensure that the `fuelup toolchain install` command accepts the following as valid inputs:

- nightly
- beta-1
- beta-2
- latest with date
- nightly with date

We could also test that we reject invalid inputs:

- latest with date, non-existent date (published dates are [here](https://github.com/FuelLabs/fuelup/tree/gh-pages/channels/latest))
- beta with date