Add in-tree buildkite pipeline

ci/test-stable.sh

@@ -0,0 +1,11 @@
+#!/bin/bash -e
+
+cd $(dirname $0)/..
+
+source $HOME/.cargo/env
+rustup component add rustfmt-preview
+cargo fmt -- --write-mode=diff
+cargo build --verbose
+cargo test --verbose
+
+exit 0