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

[spiral-ladder]

We have 3 tests that actually attempt to install a toolchain in 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)
• beta with date