Rewrite REPL test parser

- Use a recursive descent parser so that it's easy to extend.
- Add `@args` to enable customizing command-line arguments
- Add `@should-start` to enable `nix repl` tests that error before
  entering the REPL
- Make sure to read all stdout output before comparing. This catches
  some extra output we were tossing out before!

Change-Id: I5522555df4c313024ab15cd10f9f04e7293bda3a

tests/functional/repl_characterization/data/basic_tidied.test

@@ -0,0 +1,10 @@
+command
+output output one
+
+output output two
+command two
+output output output
+output output output
+
+command three
+meow output