test-template-engine: do not evaluate unused keywords by custom engine If the custom engine, "mytemplater", were installed as the default, it would enter to an infinite recursion at stringify(v) because template keywords may generate a nested mapping containing the same keywords. Spotted by a future patch which will replace context.resource('templ')(...) with context.process(...).

py3: make test-template-engine.t bytes-safe

test-template-engine: deduplicate methods of custom template engine