tests: fix clang-tidy pointed out type incompatibility

tests/testing/TimeoutAqlTest.cpp

@@ -47,7 +47,7 @@ std::string readFromFD ( int fd ) {
 	static const size_t BUFSIZE = 64;
 	std::string res;
-	int rd;
+	ssize_t rd;
 	std::array < char, BUFSIZE > buf;
 	while ( ( rd = read ( fd, buf.data ( ), buf.size ( ) - 1 ) ) > 0 ) {