use SvtSysLocaleOptions::GetRealUILanguageTag(), not GetLocaleConfigString()

Assuming that the screenshots directory suffix
* should match the configured UI locale instead of the work locale
* should not be empty if configuration says "Default - ..."
  * actually ScreenshotTest::implSaveScreenshot() explicitly tests
    (maCurrentLanguage == "en-US") and then sets the suffix to empty, so no
    other locale should exhibit the same

Change-Id: Ie4561315b67c3ade20f3947772af45e471d0752f

test/source/screenshot_test.cxx

@@ -40,7 +40,7 @@ ScreenshotTest::ScreenshotTest()
     maKnownDialogs()
 {
     SvtSysLocaleOptions localeOptions;
-    maCurrentLanguage = localeOptions.GetLocaleConfigString();
+    maCurrentLanguage = localeOptions.GetRealUILanguageTag().getBcp47();
 }
 
 ScreenshotTest::~ScreenshotTest()