test: add headless test framework

scripts/test

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+set -u
+
+usage() {
+    cat <<EOF
+Usage: ${0##*/} [--help] [TARGET ...]
+
+Run Neovim integration tests in a single 'nvim --headless' instance.
+
+With no targets, runs every test/**/*_test.lua. Each TARGET may be a
+test file or a directory; directories expand to all _test.lua files
+beneath them.
+
+Exit status is 0 if all tests passed, non-zero otherwise.
+EOF
+}
+
+if [[ ${1:-} == --help || ${1:-} == -h ]]; then
+    usage
+    exit 0
+fi
+
+cd "$(dirname "$0")/.." || exit 1
+
+if [[ -t 1 ]]; then
+    export TEST_COLOR=1
+fi
+
+exec nvim --headless --clean -l test/runner.lua "$@"