Revert serial_test back to 2.0.

Author: sunfishcode

Cargo.toml

@@ -71,7 +71,7 @@ default-features = false
 tempfile = "3.5.0"
 libc = "0.2.171"
 libc_errno = { package = "errno", version = "0.3.10", default-features = false }
-serial_test = "3"
+serial_test = "2.0.0"
 memoffset = "0.9.0"
 flate2 = "1.0"
 static_assertions = "1.1.0"