sim: msp430: set initial PC to ELF entry if available

If we want to run a simple ELF, the reset vector isn't set up, so starting
at address 0 doesn't make sense.  Use the ELF's entry point instead.
sim: msp430: start a test framework

The current sim lacks any sort of tests.  Start a basic framework and
add a simple one to test the add insn.