edify: Rename parse_string to ParseString and let it take std::string.

Also simplify the helper function expect() in {edify,updater}_test.cpp.

Test: Run recovery_component_test on marlin.
Change-Id: If54febba4b5013f6d71546318a1ca6b635204ac8
diff --git a/updater/updater.cpp b/updater/updater.cpp
index 40e3f1f..e06d453 100644
--- a/updater/updater.cpp
+++ b/updater/updater.cpp
@@ -134,7 +134,7 @@
 
   std::unique_ptr<Expr> root;
   int error_count = 0;
-  int error = parse_string(script.c_str(), &root, &error_count);
+  int error = ParseString(script, &root, &error_count);
   if (error != 0 || error_count > 0) {
     LOG(ERROR) << error_count << " parse errors";
     CloseArchive(za);