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