chore: re-enable test_socket_pair on FreeBSD (#3844)

It was disabled due to an OS bug that has been fixed on all currently
supported releases of FreeBSD

tokio/tests/uds_cred.rs

@@ -8,10 +8,6 @@ use libc::getegid;
 use libc::geteuid;
 
 #[tokio::test]
-#[cfg_attr(
-    target_os = "freebsd",
-    ignore = "Requires FreeBSD 12.0 or later. https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=176419"
-)]
 #[cfg_attr(
     target_os = "netbsd",
     ignore = "NetBSD does not support getpeereid() for sockets created by socketpair()"