add shellspec golden harness for the legacy make engine

- spec/golden/cases.txt: 69 black-box cases (CLI wrapper mode and make
  include mode) run in a hermetic sandbox with a mocked docker
- spec/golden/expected/*.txt recorded from this engine (baseline)
- make test / test-golden / golden-record / lint dev targets

Makefile

@@ -1 +1,23 @@
+DEV_TARGETS := test test-unit test-golden test-integration lint golden-record
+ifneq ($(filter $(DEV_TARGETS),$(MAKECMDGOALS)),)
+
+SHELLSPEC ?= shellspec
+SHELLCHECK ?= shellcheck
+
+.PHONY: $(DEV_TARGETS)
+test: ## Run unit + golden tests
+	$(SHELLSPEC)
+test-unit: ## Run unit tests only
+	$(SHELLSPEC) spec/unit
+test-golden: ## Run golden tests only (MYOS_ENGINE=legacy|cli)
+	$(SHELLSPEC) spec/golden
+test-integration: ## Run tests needing a real docker daemon
+	$(SHELLSPEC) --tag integration
+golden-record: ## Re-record golden expectations from the legacy engine
+	spec/golden/record.sh $(CASES)
+lint: ## shellcheck all shell sources
+	$(SHELLCHECK) -s bash myos spec/golden/record.sh spec/support/run.sh spec/support/bin/* $(wildcard bin/* lib/*.sh lib/cmd/*.sh install.sh)
+
+else
 include make/include.mk
+endif