Add tests for every implemented command

tests/service_alias.bats

@@ -0,0 +1,32 @@
+#!/usr/bin/env bats
+load test_helper
+
+setup() {
+  dokku "$PLUGIN_COMMAND_PREFIX:create" l >&2
+}
+
+teardown() {
+  dokku --force "$PLUGIN_COMMAND_PREFIX:destroy" l >&2
+}
+
+@test "($PLUGIN_COMMAND_PREFIX:alias) error when there are no arguments" {
+  run dokku "$PLUGIN_COMMAND_PREFIX:alias"
+  assert_contains "${lines[*]}" "Please specify a name for the service"
+}
+
+@test "($PLUGIN_COMMAND_PREFIX:alias) error when alias is missing" {
+  run dokku "$PLUGIN_COMMAND_PREFIX:alias" l
+  assert_contains "${lines[*]}" "Please specify an alias for the service"
+}
+
+@test "($PLUGIN_COMMAND_PREFIX:alias) error when service does not exist" {
+  run dokku "$PLUGIN_COMMAND_PREFIX:alias" not_existing_service MY_ALIAS
+  assert_contains "${lines[*]}" "service not_existing_service does not exist"
+}
+
+@test "($PLUGIN_COMMAND_PREFIX:alias) success" {
+  run dokku "$PLUGIN_COMMAND_PREFIX:alias" l MY_ALIAS
+  new_alias=$(cat "$PLUGIN_DATA_ROOT/l/ALIAS")
+  [[ $new_alias == "MY_ALIAS" ]]
+}
+