Create "Undo" unit test

[mortbopet]

A unit test for testing the "undo" feature of a vsrtl::Design should be added. The test should verify that:

• the undo feature correctly reverts the circuit to a previous state.
• forcing a register value does not change the undo stack state.
• if clock cycles < undo stack size, the circuit should be undo-able unto the initial state
• if clock cycles > undo stack size, the circuit should be undo-able #undo stack size times