[libcalamares] Use more descriptive variable name

- If the test failed, you'd get a cryptic message like
	FAIL!  : NetworkTests::testPing() 'r' returned FALSE. ()
   So rename the variable so the failure mode is more obvious.
   (Could have used QVERIFY2() instead, this is simpler)

src/libcalamares/network/Tests.cpp

@@ -47,6 +47,6 @@ NetworkTests::testPing()
     using namespace CalamaresUtils::Network;
     Logger::setupLogLevel( Logger::LOGVERBOSE );
     auto& nam = Manager::instance();
-    auto r = nam.synchronousPing( QUrl( "https://www.kde.org" ), RequestOptions( RequestOptions::FollowRedirect ) );
-    QVERIFY( r );
+    auto canPing_www_kde_org = nam.synchronousPing( QUrl( "https://www.kde.org" ), RequestOptions( RequestOptions::FollowRedirect ) );
+    QVERIFY( canPing_www_kde_org );
 }