Allow testing systemd integration with a test unit file

tray/testfiles/syncthing-testinstance.service

@@ -0,0 +1,32 @@
+# Unit file for testing systemd integration with a test instance of Syncthing
+# 1. Copy to ~/.config/systemd/user/syncthing-testinstance.service for using it.
+# 2. Set HOME=/tmp/syncthingtest when launching Syncthing Tray itself to be in
+#    accordance with this unit file.
+# 3. Set SYNCTHINGTRAY_SYSTEMD_USER_UNIT=syncthing-testinstance.service so the
+#    wizard will make use of this unit instead of the normal one.
+
+[Unit]
+Description=Test instance of Syncthing with HOME under /tmp/syncthingtest
+Documentation=man:syncthing(1)
+StartLimitIntervalSec=60
+StartLimitBurst=4
+
+[Service]
+ExecStart=/usr/bin/syncthing serve --no-browser --no-restart --logflags=0
+Restart=on-failure
+RestartSec=1
+SuccessExitStatus=3 4
+RestartForceExitStatus=3 4
+Environment=HOME=/tmp/syncthingtest
+
+# Hardening
+SystemCallArchitectures=native
+MemoryDenyWriteExecute=true
+NoNewPrivileges=true
+
+# Elevated permissions to sync ownership (disabled by default),
+# see https://docs.syncthing.net/advanced/folder-sync-ownership
+#AmbientCapabilities=CAP_CHOWN CAP_FOWNER
+
+[Install]
+WantedBy=default.target

widgets/settings/setupdetection.cpp

@@ -1,5 +1,8 @@
 #include "./setupdetection.h"
 
+// use meta-data of syncthingtray application here
+#include "resources/../../tray/resources/config.h"
+
 #if defined(LIB_SYNCTHING_CONNECTOR_SUPPORT_SYSTEMD) && (defined(PLATFORM_UNIX) || defined(PLATFORM_MINGW) || defined(PLATFORM_CYGWIN))
 #define PLATFORM_HAS_GETLOGIN
 #include <unistd.h>
@@ -11,8 +14,17 @@ SetupDetection::SetupDetection(QObject *parent)
     : QObject(parent)
 {
     // assume default service names
+    const auto defaultUserUnit =
+#if QT_VERSION >= QT_VERSION_CHECK(5, 10, 0)
+        qEnvironmentVariable(PROJECT_VARNAME_UPPER "_SYSTEMD_USER_UNIT",
+#endif
+            QStringLiteral("syncthing.service")
+#if QT_VERSION >= QT_VERSION_CHECK(5, 10, 0)
+        )
+#endif
+        ;
 #ifdef LIB_SYNCTHING_CONNECTOR_SUPPORT_SYSTEMD
-    userService.setUnitName(QStringLiteral("syncthing.service"));
+    userService.setUnitName(defaultUserUnit);
     systemService.setUnitName(QStringLiteral("syncthing@") %
 #ifdef PLATFORM_HAS_GETLOGIN
         QString::fromLocal8Bit(getlogin()) %