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/no_nested_debuggers.test

@@ -1,3 +1,4 @@
+@args --debugger
 we enter a debugger via builtins.break in the input file.
 
   info: breakpoint reached
@@ -37,3 +38,4 @@ and once again, more breakpoints are ignored.
 
   nix-repl> builtins.break 3
   3
+  error: bar