Removed UI-dependency of CBN that caused some tests to fail in NUnit GUI

On NUnit GUI some tests (particularly in ```StringTests```) failed because ```CodeBlockNode``` tries to access UI resources and causes exception to be thrown. This submission prevents that by removing unnecessary UI access from ```CodeBlockNode```.

This makes some test cases pass on NUnit GUI, but not ```nunit-console.exe``` (therefore the results on CI remains unchanged). I still need to investigate the root cause of it later. This submission just makes NUnit GUI behaves properly, at least.