Disable another wxSTC unit test mysteriously failing under Xvfb

For some reason, the control does lose focus there, even if it doesn't
happen when using a normal X11 server.

Just disable the test in this environment.

tests/controls/styledtextctrltest.cpp

@@ -151,7 +151,11 @@ TEST_CASE_METHOD(StcPopupWindowsTestCase,
         m_stc->CallTipCancel();
 
     // Verify that clicking the call tip did not take focus from the STC.
-    CHECK_FOCUS_IS( m_stc );
+    //
+    // Unfortunately this test fails for unknown reasons under Xvfb (but only
+    // there).
+    if ( !IsRunningUnderXVFB() )
+        CHECK_FOCUS_IS( m_stc );
 
     // With wxGTK there is the same problem here as in the test above.
 #ifndef __WXGTK__