[C++] add test to show that display string can be printed at any point during the decode of a message.
[C++] add test to show that display string can be printed at any point during the decode of a message.

PKRoma/simple-binary-encoding,real-logic/simple-binary-encoding,real-logic/simple-binary-encoding,real-logic/simple-binary-encoding,PKRoma/simple-binary-encoding,real-logic/simple-binary-encoding,PKRoma/simple-binary-encoding,real-logic/simple-binary-encoding,PKRoma/simple-binary-encoding,real-logic/simple-binary-encoding,PKRoma/simple-binary-encoding,real-logic/simple-binary-encoding,PKRoma/simple-binary-encoding,PKRoma/simple-binary-encoding