put feature(test) behind unstable feat flag so compiling with stable works

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "influx-writer"
-version = "0.19.1"
+version = "0.19.2"
 authors = ["Jonathan Strong <jonathan.strong@gmail.com>"]
 edition = "2018"
 description = "opinionated influxdb client"

src/lib.rs

@@ -1,9 +1,9 @@
 //! Utilities to efficiently send data to influx
 //!
 
-#![feature(test)]
+#![cfg_attr(feature = "unstable", feature(test))]
 
-#[cfg(test)]
+#[cfg(all(feature = "unstable", test))]
 extern crate test;
 #[macro_use]
 extern crate slog;