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/edify/parser.yy b/edify/parser.yy
index bd2e010..3a63c37 100644
--- a/edify/parser.yy
+++ b/edify/parser.yy
@@ -138,7 +138,7 @@
   ++*error_count;
 }
 
-int parse_string(const char* str, std::unique_ptr<Expr>* root, int* error_count) {
-    yy_switch_to_buffer(yy_scan_string(str));
-    return yyparse(root, error_count);
+int ParseString(const std::string& str, std::unique_ptr<Expr>* root, int* error_count) {
+  yy_switch_to_buffer(yy_scan_string(str.c_str()));
+  return yyparse(root, error_count);
 }