tests: retry slice cache warm-up during nginx startup

Author: zhuizhuhaomeng

t/190-http2-slice-subreq-timeout.t

@@ -174,7 +174,10 @@ system("dd", "if=/dev/urandom", "of=$file", "bs=1M", "count=2", "status=none") =
     or die "failed to create $file";
 
 my $port = $Test::Nginx::Util::ServerPortForClient;
+# Valgrind starts nginx asynchronously and can delay the listener under load.
 my $cmd = "curl -sS --connect-timeout 5 --max-time 30 "
+          . "--retry 30 --retry-connrefused --retry-delay 1 "
+          . "--retry-max-time 30 "
           . "http://127.0.0.1:$port/slice.bin -o /dev/null";
 system($cmd) == 0 or die "failed to warm sliced proxy cache: $cmd";