Skip another test using /proc and /sys when using LXC

This should have been part of the previous commit.

tests/filename/filenametest.cpp

@@ -1053,6 +1053,9 @@ void FileNameTestCase::TestShortcuts()
 // Check that GetSize() works correctly for special files.
 TEST_CASE("wxFileName::GetSizeSpecial", "[filename][linux][special-file]")
 {
+    if ( IsRunningInLXC() )
+        return;
+
     wxULongLong size = wxFileName::GetSize("/proc/kcore");
     INFO( "size of /proc/kcore=" << size );
     CHECK( size > 0 );