Re-enable tests on driver crates

[wmmc88]

Previously, due to rust-lang/cargo#12663 (comment), driver crates had to have tests disabled. Since that bug has been fixed in cargo, we can now re-enable unit tests in crates. This also removes the need for having a separate lib crate to contain driver tests.

• re-enable tests in driver crates
• update docs to remove recommendations to have a separate lib crate to enable unit tests

[wmmc88]

Since this change just got merged into cargo, we won't see this change until the latest cargo is ingested into rust and a new stable release is made. I think that this will be earliest available at 1.87 (5 May, 2025)