Re-enable pool tests
The pool tests have been implicitly disabled by testing
for a feature that has been removed.
-#[cfg(all(test, feature = "smtp-transport", feature = "r2d2"))]
+#[cfg(all(test, feature = "smtp-transport", feature = "pool"))]
mod sync {
use lettre::{address::Envelope, SmtpTransport, Transport};
use std::{sync::mpsc, thread};
