Skip wxTextCtrl::HitTest() unit test failing under Xvfb

For some reason, wxTextCtrl scrolls much too far when running under
Xvfb, so the test assumption that it still has some visible characters
after calling SetInsertionPointEnd() fails and must be disabled in this
environment.

tests/controls/textctrltest.cpp

@@ -377,8 +377,21 @@ void TextCtrlTestCase::HitTestSingleLine()
         // wxGTK must be given an opportunity to lay the text out.
         wxYield();
 
-        REQUIRE( m_text->HitTest(wxPoint(2*sizeChar.x, yMid), &pos) == wxTE_HT_ON_TEXT );
-        CHECK( pos > 3 );
+        // For some reason, this test consistently fails when running under
+        // Xvfb. Debugging shows that the text gets scrolled too far, instead
+        // of scrolling by ~156 characters, leaving the remaining 44 shown, in
+        // normal runs, it gets scrolled by all 200 characters, leaving nothing
+        // shown. It's not clear why does it happen, and there doesn't seem
+        // anything we can do about it.
+        if ( IsRunningUnderXVFB() )
+        {
+            WARN("Skipping test known to fail under Xvfb");
+        }
+        else
+        {
+            REQUIRE( m_text->HitTest(wxPoint(2*sizeChar.x, yMid), &pos) == wxTE_HT_ON_TEXT );
+            CHECK( pos > 3 );
+        }
     }
 #endif
 }