tests: support env-backed smoke passwords

tests/login_smoke.cpp

@@ -1,5 +1,6 @@
 #include <array>
 #include <cerrno>
+#include <cstdlib>
 #include <cstdint>
 #include <cstring>
 #include <iostream>
@@ -381,13 +382,28 @@ int main(int argc, char** argv)
 {
 	try
 	{
-		Expect(argc == 6, "usage: metin_login_smoke <host> <auth_port> <channel_port> <login> <password>");
+		Expect(argc == 6,
+		       "usage: metin_login_smoke <host> <auth_port> <channel_port> <login> <password>\n"
+		       "   or: metin_login_smoke <host> <auth_port> <channel_port> <login> --password-env=ENV_NAME");
 
 		const std::string host = argv[1];
 		const uint16_t auth_port = static_cast<uint16_t>(std::stoi(argv[2]));
 		const uint16_t channel_port = static_cast<uint16_t>(std::stoi(argv[3]));
 		const std::string login = argv[4];
-		const std::string password = argv[5];
+		std::string password;
+
+		const std::string password_arg = argv[5];
+		const std::string prefix = "--password-env=";
+		if (password_arg.rfind(prefix, 0) == 0)
+		{
+			const char* value = std::getenv(password_arg.substr(prefix.size()).c_str());
+			Expect(value && *value, "password environment variable is empty");
+			password = value;
+		}
+		else
+		{
+			password = password_arg;
+		}
 
 		Expect(login.size() <= LOGIN_MAX_LEN_LOCAL, "login too long");
 		Expect(password.size() <= PASSWD_MAX_LEN_LOCAL, "password too long");