Add environment variable check for tst_ProxyPac::proxypac()

Yield error message if --proxy-pac-url argument is not passed.

Change-Id: I9a5685982c0a9d8decc65ad9c728357c038ef0a0
Reviewed-by: Jüri Valdmann <juri.valdmann@qt.io>

tests/auto/widgets/proxypac/tst_proxypac.cpp

@@ -46,6 +46,10 @@ private slots:
 
 void tst_ProxyPac::proxypac()
 {
+    const QString fromEnv = QString::fromLocal8Bit(qgetenv("QTWEBENGINE_CHROMIUM_FLAGS"));
+    if (!fromEnv.contains("--proxy-pac-url"))
+        qFatal("--proxy-pac-url argument is not passed.");
+
     ProxyServer proxyServer1;
     proxyServer1.setPort(5551);
     proxyServer1.run();