Split up minidump register context tests

The tests were doing two somewhat independent things:
- checking that the registers can be retrieved from the minidump file
- checking that they can be converted into a form suitable for
  consumption by lldb

The first thing requires a minidump file (but it's independent of other
lldb structures), while the second one does not require a minidump file
(but it needs lldb register info structures).

Splitting this into two tests gives an opportunity to write more
detailed tests, and allows the two pieces of functionality to be moved
into different packages, if that proves to be necessary.

llvm-svn: 354662