Rename lslImmediate tests

Remove the redundancy from the test case names.
Access context registers via setter/getter

This allows for future updates to allow switching between the MSP and
the PSP.
Add LSR Immediate

I also fixed the use of an incorrect bitmask in
shiftAddSubtractMoveCompare() that was uncovered when I started
decoding lsr instructions as well.  I also added 2 more test cases to
the LSL immediate suite to make sure that I covered carry out from both
shift extremes to make sure that my math was correct for carryOut
determination.