Add local testing Makefile

Use `make test` to run test (no pun intended) for this library.

Use `make autotest` (thanks to watchexec dependency) to run test continuously
when introducing changes to either test or gem files.

Kept this ignored a long time, use it locally during development and
thought was good to have it part of the official repository.

For `autotest` to work, you will need the external `watchexec` dependency:
https://github.com/mattgreen/watchexec

[skip ci]

Makefile

@@ -0,0 +1,14 @@
+RUBY ?= ruby # default name for Ruby interpreter
+
+.PHONY: default
+default: test
+
+# `autotest` task uses `watchexec` external dependency:
+# https://github.com/mattgreen/watchexec
+.PHONY: autotest
+autotest:
+	watchexec --exts rb --watch lib --watch test --clear "$(RUBY) -S rake test"
+
+.PHONY: test
+test:
+	$(RUBY) -S rake test