SOLR-17871: fix error-prone warning

Author: dsmiley

solr/solrj/src/test/org/apache/solr/client/solrj/impl/Http2SolrClientTest.java

@@ -712,7 +712,7 @@ public void testIdleTimeoutWithHttpClient() throws Exception {
 
       // too little time to succeed
       int packets = LuceneTestCase.RANDOM_MULTIPLIER == 1 ? 10 : 80; // 60 crosses a default timeout
-      long timeToSendMs = packets * BasicHttpSolrClientTest.SlowStreamServlet.PACKET_MS;
+      long timeToSendMs = (long) packets * BasicHttpSolrClientTest.SlowStreamServlet.PACKET_MS;
       QueryRequest req = new QueryRequest(SolrParams.of("count", "" + packets));
       req.setResponseParser(new InputStreamResponseParser(FILE_STREAM));
       assertIsTimeout(expectThrows(SolrServerException.class, () -> oldClient.request(req)));