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

edify/include/edify/expr.h

@@ -156,6 +156,6 @@ Value* StringValue(const char* str);
 
 Value* StringValue(const std::string& str);
 
-int parse_string(const char* str, std::unique_ptr<Expr>* root, int* error_count);
+int ParseString(const std::string& str, std::unique_ptr<Expr>* root, int* error_count);
 
 #endif  // _EXPRESSION_H

edify/parser.yy

@@ -138,7 +138,7 @@ void yyerror(std::unique_ptr<Expr>* root, int* error_count, const char* s) {
   ++*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);
 }