Support plain `cargo test` and disable unused doctests for speed

Support plain `cargo test` and disable unused doctests for speed

Since DB tests execute diesel migrations automatically, concurrent
execution causes flaky failures from simultaneous migrations. This can
be worked around using `cargo test --workspace -- --test-threads=1`,
which is what the CI config does, but this is not intuitive for
newcomer developers and unnecessarily slows down the test suite for
the majority of tests which are safe to run concurrently. This fixes
this issue by integrating with the small test crate `serial_test` and
using it to explicitly mark DB tests to run sequentially while
allowing all other tests to run in parallel.

Additionally, this greatly improves the speed of `cargo test` by
disabling doc-tests in all crates, since these are aren't currently
used and cargo's doc-test pass, even when no doc-tests exist, has
significant overhead. On my machine, this change significantly
improves test suite times by about 85%, making it much more practical
to develop with tools like `cargo watch` auto-running tests.