Fix the timing issue of QFtp tests when using Docker servers

Sometimes, it fails in tst_QFtp::proxy or tst_QFtp::activeMode under the
stress test. It complains about "Network operation timed out" on
vsftpd.test-net.qt.local.

When this issue happens in tst_QFtp::proxy, it shows "USER-ERR reject:
172.18.0.5 (ForkLimit 40)" in the log of ftp-proxy. By default, the
ftp-proxy only supports 40 incoming client connections per minute. To
make the ftp-proxy more powerful, this change extends the limits from 40
to 2000.

When this issue happens in tst_QFtp:activeMode, vsftpd shows 426 Failure
writing network stream error. It is a known issue of vsftpd. A quick fix
is turning use_sendfile off.

Change-Id: Iad50469654041bf30f92ef00805034f0d4aa9c3f
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

tests/testserver/ftp-proxy/ftp-proxy.sh

@@ -34,7 +34,9 @@ set -ex
 # package ftp-proxy
 
 # install configurations and test data
-sed -i 's/# AllowMagicUser\tno/AllowMagicUser\tyes/' /etc/proxy-suite/ftp-proxy.conf
+sed -i -e 's/# AllowMagicUser\tno/AllowMagicUser\tyes/' \
+    -e 's/# ForkLimit\t\t40/ForkLimit\t\t2000/' \
+    /etc/proxy-suite/ftp-proxy.conf
 
 # enable service with installed configurations
 ftp-proxy -d

tests/testserver/vsftpd/testdata/vsftpd.conf

@@ -101,6 +101,10 @@ userlist_enable=YES
 listen=YES
 tcp_wrappers=YES
 
+# An internal setting used for testing the relative benefit of using the
+# sendfile() system call on your platform.
+use_sendfile=NO
+
 # Enabling SFTP
 #ssl_enable=YES
 #allow_anon_ssl=YES