Added unit tests for cmd_continue

Code coverage for cmd_continue.cpp is now 100%.

I added plaformMocks to help in writing and verifying these unit tests.
I also defaulted the Semihost_IsDebuggeeMakingSemihostCall() stub to
return FALSE by default instead of TRUE since most tests want to make
it through to the GDB command handling loop in mri.c
Added unit tests for cmd_step module

Code coverage for this module is now 100%.

I added some single stepping related functionality to the Platform
Mocking layer.

I also set the initial platform mock controlled program counter to
INITIAL_PC (0x10000000).