Inlined format args make code more readable, and code more compact.
I ran this clippy command to fix most cases, and then cleaned up a few trailing commas and uncaught edge cases.
```
cargo clippy --bins --examples --benches --tests --lib --workspace --fix -- -A clippy::all -W clippy::uninlined_format_args
```
If the first test thread is a bit slow after it acquires the
`DO_CLEANUP` permit, it can accidentally delete a fresh test db created
by another thread right before that other thread has a chance to open
its connection.
This fix simply records the current timestamp _before_ we acquire the
`DO_CLEANUP` permit and only cleans up test db's created before then.
