peg-tests.el: Fix test failures

peg-tests.el: Fix test failures

* lisp/progmodes/peg.el (peg-parse): Refine heuristic since unknown
terminals are resolved at run-time rather than compile-time now.
(peg--macroexpand) <stack-action>: Avoid generating a `let` with an
empty body.
(peg--translate-rule-body): Adjust to name change of
`macroexp-warn-and-return` and the fact that it's always available.

* test/lisp/progmodes/peg-tests.el (peg-parse-string):
Add `indent` declaration.
(peg-test): Check that the compiler emits the warnings we expect.