Respect DISPLAY environment variable in XWindows tests

XWindowsScreenTests was failing under Gentoo's package manager. I have
also addressed this in XWindowsScreenSaverTests, which is currently
unused.

src/test/integtests/platform/XWindowsScreenSaverTests.cpp

@@ -23,6 +23,7 @@
 #include "platform/XWindowsScreenSaver.h"
 
 #include "test/global/gtest.h"
+#include <cstdlib>
 #include <X11/Xlib.h>
 
 using ::testing::_;
@@ -30,7 +31,12 @@ using ::testing::_;
 // TODO: not working on build machine for some reason
 TEST(CXWindowsScreenSaverTests, activate_defaultScreen_todo)
 {
-    Display* display = XOpenDisplay(":0.0");
+    const char* displayName = std::getenv("DISPLAY");
+    if (displayName == NULL) {
+        displayName = ":0.0";
+    }
+
+    Display* display = XOpenDisplay(displayName);
     Window window = DefaultRootWindow(display);
     MockEventQueue eventQueue;
     EXPECT_CALL(eventQueue, removeHandler(_, _)).Times(1);

src/test/integtests/platform/XWindowsScreenTests.cpp

@@ -20,16 +20,22 @@
 #include "platform/XWindowsScreen.h"
 
 #include "test/global/gtest.h"
+#include <cstdlib>
 
 using ::testing::_;
 
 TEST(CXWindowsScreenTests, fakeMouseMove_nonPrimary_getCursorPosValuesCorrect)
 {
+    const char* displayName = std::getenv("DISPLAY");
+    if (displayName == NULL) {
+        displayName = ":0.0";
+    }
+
     MockEventQueue eventQueue;
     EXPECT_CALL(eventQueue, adoptHandler(_, _, _)).Times(2);
     EXPECT_CALL(eventQueue, adoptBuffer(_)).Times(2);
     EXPECT_CALL(eventQueue, removeHandler(_, _)).Times(2);
-    XWindowsScreen screen(new XWindowsImpl(), ":0.0", false, false, 0, &eventQueue);
+    XWindowsScreen screen(new XWindowsImpl(), displayName, false, false, 0, &eventQueue);
 
     screen.fakeMouseMove(10, 20);