tests: use static cast in timeout executor

tests/testing/TimeoutAqlTest.cpp

@@ -57,7 +57,7 @@ std::string readFromFD ( int fd ) {
 }
 void writeToFD ( int fd, const std::string & message, const std::string & errorDesc ) {
-	if ( write ( fd, message.c_str ( ), message.length ( ) ) != ( ssize_t ) message.length ( ) )
+	if ( write ( fd, message.c_str ( ), message.length ( ) ) != static_cast < ssize_t > ( message.length ( ) ) )
 		throw std::runtime_error ( "TimeoutAqlTest: write() to pipe failed (" + errorDesc + ")" );
 }