add tests for completions

test/completions.bats

@@ -0,0 +1,48 @@
+#!/usr/bin/env bats
+
+load test_helper
+
+create_command() {
+  bin="${RBENV_TEST_DIR}/bin"
+  mkdir -p "$bin"
+  echo "$2" > "${bin}/$1"
+  chmod +x "${bin}/$1"
+}
+
+@test "command with no completion support" {
+  create_command "rbenv-hello" "#!$BASH
+    echo hello"
+  run rbenv-completions hello
+  assert_success ""
+}
+
+@test "command with completion support" {
+  create_command "rbenv-hello" "#!$BASH
+# provide rbenv completions
+if [[ \$1 = --complete ]]; then
+  echo hello
+else
+  exit 1
+fi"
+  run rbenv-completions hello
+  assert_success "hello"
+}
+
+@test "forwards extra arguments" {
+  create_command "rbenv-hello" "#!$BASH
+# provide rbenv completions
+if [[ \$1 = --complete ]]; then
+  shift 1
+  while [[ \$# -gt 0 ]]; do
+    echo \$1
+    shift 1
+  done
+else
+  exit 1
+fi"
+  run rbenv-completions hello happy world
+  assert_success
+  assert_line 0 "happy"
+  assert_line 1 "world"
+  refute_line 2
+}